What is this painful skin condition?

Well, clearly you should see a dermatologist. Sounds like it could be Hidradonitis suppertiva. There are several stages of H.S. from little bumps to really horrible big ones. Also, it typically forms on inner thighs, groin area, armpits and under the . I would definately ask the dermatologist about the possiblity (and/or do some research on the internet). Good luck!