Didn't you have a sartorius injury before? Is this the same area? That and/or maybe the adductor magnus, if it's bugging you above the medial knee?

I'm guessing just a bit of overuse if you've been increasing mileage lately. Some of those adductor muscles don't see a lot of use off the bike. Easy does it. Day or two off is probably a good idea. Don't neglect the medial thighs with your foam roller. And half moon pose seems to cure everything for me...