What Causes Rashes On The Chest?

I recently went to the doctor because I have two round red spots/rash on my chest or more so between my chest and my ribs. The doctor had me tested for Lyme disease. I got my results yesterday and it said that I don't have Lyme disease. What are the chances of the test being inaccurate. Since going to the doctor i have had pain in my knees and elbows.