Bug bite on back of thigh for 4 days. it's a very defined puffy red circle with dark brown center. doesn't hurt unless we touch it. dr appt necessary?

Could be developing: Brown Recluse Spider bite. Toxin causes tissue death and open ulcer formation which is at risk for secondary skin infection. You should go see your doc,if not improving or keeps getting worse. Take daily pictures ,in good light ,to judge progress.

Depends: If it is getting better you can watch and wait. If the circle is enlarging (mark it with a pen to follow it) or if it is getting more painful then you should be seen.