What's your flexibility like? Can you get your heel to your butt? What's it feel like to heel-sit?
In all liklihood you have a strain ... it should get better with stretching ... and I hate to tell you, but you need to foam roll (you personally ... the fact that is it so painful says you need it!). If you can't take a foam roller, then try using a rolling pin (wood or plastic) while sitting with your legs out in front. Or you could buy
"The Stick" as it is a little more gentle than a foam roller if you are really sore.
Lay off the lunges and stuff for a week or so, and stick to stretching and rolling, along with hamstring and gluteal activation and strengthening stuff (SHELC, bridges, GHRs, etc)
HTH!