A couple years ago I noticed a ring shaped rash developing on my leg. After it grew from 2in to 4in I grew concerned. I visited my usual doctor and she said it was just ring worm. She prescribed a ridiculously large tube of anti-itch cream and sent me on my way. I knew she was wrong for several reasons:
1. I've had ring worm before and this was definitely not ringworm. It was not blistery or oozy.
2. It did not itch but she prescribed anti-itch cream anyways.
3. She did not instruct me to treat it with anti-fungal like a doctor who knew what they were doing would have done.
I got a second opinion from a friend who is a rheumatoid arthritis doctor who specializes in lyme disease. By this time the rash was larger and another ring was forming in the center. He took one look at it and gave me the correct diagnosis of lyme disease. I had not developed other symptoms like arthritis and did not have the anti-body, but he is an expert on the subject and gives lectures around the country about lyme disease, so I trust him. He gave me a round of antibiotics and everything cleared up.
Moral of the story: if you feel that the doctor isn't right, get a second opinion!