Having read some of the other posts on this forum, I thought I should share my experience as it may provide some comfort to people in a similar situation.
A bit of background - I am a 25 years old male, a professional engineer, into running, football, surfing etc. Never really had any medical complaints save for the odd broken bone.
The problems started in september 2007 - I had been surfing during the day, came back knackered and went to bed. I never really got to sleep, just lay there for a couple of hours feeling agitated and drifting in and out of consciousness. Some time in the middle of the night I finally drifted off but woke suddenly with a pain in my chest and the feeling of my heart rate accelarating. It was accompanied by a strange hot flushing sensation, and reached a crescendo after a couple of minutes. I felt pretty dizzy and sick, and was pretty scared - I had 999 dialled into my phone but didn't press 'call'. I decided to drive myself to hospital where they ran some tests and failed to find anything wrong. The following day I felt like I had flu, all shivery and weak, worried about my symptoms and generally feeling like crap.
Over the next couple of weeks I carried on running but noticed that as my heart rate accelarated it would miss or insert beats, causing that 'butterflies in your chest' sensation, and also that it would max out at what seemed an unhealthily high rate. A couple of times when I was sitting down or asleep I got the same hot flush sensation and never really seemed to recover from the flu symptoms.
About a month later, I was crammed in the back of a car with some mates, and without any warning I got a series of really powerful thumps and flutters in my chest, it felt like something was trying to escape...pain spread from my chest to my shoulder, the hotness and dizziness came back - a proper 'this is it' moment. But again it subsided, and I just spent the evening feeling frail and shivery. By this point I had had several appointments with my GP, who had tested me for a bunch of stuff. The gap between the tests seemed like an eternity - I was desparate to find what was wrong, not knowing being by far the worst thing. I had a 24 hour portable ecg hooked up to me - came back negative. I wasnt really exercising any more for fear of doing some serious damage, and that was making me miserable too. Throughout december I was a wreck - getting to sleep every night took hours and it was impossible to be horizontal, my heart would go crazy and go faster, slower, up to 150bpm and down to 48bpm, skip a couple of beats, insert 3 fast ones - generally do some pretty alarming stuff for something that you normally take for granted.
I felt about 85, sleeping sitting up propped up with a pillows, missing days of work, unable to exercise as every time I tried it would start to hammer again, by this time pain throughout my digestive system as well, and still no explanation from my doctor or specialists. I turned to the internet and gained a new vocabulary of words and diseases - that only served to give me more ideas about what I might be suffering from, my symptoms matched up with a bunch of things, all serious. For someone that enjoyed life so much it was devastating that I might have something that might limit me.
Early in 2008, with all the normal tests negative, both my GP's were diagnosing anxiety as the cause of all my issues which seemed pretty ridiculous in the light of my very real symptoms. I did try and accept the instances of racing heartbeat in the middle of the night could be panic attacks - I found that during one I could actually calm myself down and the symptoms would diminish. This led me to try to accept the other symptoms as anxiety with some success, in about march I started exercising again and just ignored all the protests from my body, and surprisingly did not wake up in intensive care. This suggested that anxiety could be the underlying cause, so I had a diagnosis but still no way of getting my energy back or stopping the symptoms.
I decided that I was not enjoying my job, so after much deliberation I quit in June 2008 and went back into full time education to change my career. This was an absolute revelation as it opened my horizons to what was possible, gave me a bunch of friends and showed me how stressed and miserable I had been previously without even realising. The symptoms all seemed to evaporate, although the last to go was the digestive stuff which hung around for a while.
The upshot of this longwinded tale is that, a year later I am actually happy, with a bright future, absolutely no complaints from my body. Anxiety seems to be incredibly complicated and has an arsenal of weapons, and the resulting physical symptoms are very, very real. The whole experience was out of character for me and came without warning, but was easily the worst few months of my life.
This shouldn't stop anyone seeing a doctor given the serious symptoms, but sometimes if the entire medical profession thinks that you are fine, they are actually right. If you find yourself in a similar situation, desperate for the results of the next test and fearful of the onset of some debilitating disease, it might be worth sitting back and considering it. I have to stress that absolutely the worst, most destructive possible thing you can do is go on the internet, as it exposes you to every known disease, and the symptoms of many of the really bad ones are identical to the ones I have listed. You also have experiences of like minded people, some of whom may actually have that disease - no-one ever comes on to a medical forum to tell you how great they are feeling (that would be a bit unkind)!
And finally to those who suffer from anxiety or depression, just to say that it can and does get better, there is a light at the end of the tunnel, a future and a past, all of the things that are robbed of you while you are immersed in it. Hope this helps someone.