It's sounds like what I'm going through...I hope it's not! I have been to four doctors, including an infectious disease specialist, no one knows what it is. Last October this large knot appeared on my bikini line, right where the thigh ends, it was a discomfort, but it never did anything. My OB/GYN, who the month before removed my ovarian cyst, thought nothing of it and didn't want to open it since it didn't have a head. November, I get my general doctor to lance it. It's 90% blood, test results show nothing abnormal. Well since then it has reoccurred 3 times, each time more painful, with more puss, in the exact same spot, and all test show nothing! The doctor had do lance it the first two times, the last two it opened on it's own. Last Monday, I thought it might be flaring up, never can really tell, Tuesday & Wednesday I spent in bed on my back, Thursday it opened, right before my doctor's appointment. Tomorrow it is getting removed, I can be normal again! Hopefully I'll find out what it is and it won't ever come back!!!
Bottom-line, Listen to your body above everyone else and don't just take a doctors opinion as anything other than an educated guess. They are knowledgeable, but they can't always understand and you can't always explain what your body is saying. Oh, and please if you end up seeing multiple doctors, make sure that they are all fully informed of everything before your visit.
Once the infection clears up it should return to normal. Did he prescribe antibiotics?