What Could This Be?

(Not sure where this should go, so please moves as needed!)

Just wondering if anyone has a good guess about this???

Oldest DS (8) has been complaining for almost 3 weeks about a pain in his "leg". It is just one side right along where his underwear cuts across his leg. (Like at the bikini line, although I doubt boys call it that.)

I took him to the dr. for allergies and mentioned it in passing to the dr who checked it out. It is sore to the touch, but only seems to bother him periodically throughout the day. Doesn't seem to be "selective" - he may wince when he is having fun or engrossed in something, so I don't *think* he is faking. The dr ran basic bloodwork, and that came back normal. His white counts were a little bit low,, which the dr said can be a sign of a virus.

DH and I are wondering if it could be a pulled muscle, and if so how long it should be sore. He doesn't have any visible lumps. We are baffled, and not sure how long to give it before we take him back to the dr.

Any thoughts?