Well, let it heal. I've thankfully thanks to skating and the way I do squats (I point my toes out some, but that makes it bad for the knees because you get uneven quad development, yay...) I've got non-tight hip flexors, but lots of people got problems with their hip flexors and they're rarely stretched. Frank Yang, despite being a troll 90% of the time, made a VERY informative video about hip flexor stretches, and how they're basically the only stretch you need. So for now, well, don't squat, let things heal up. Ice/hot, whatever you wish, and in the future stretch your hip flexors out.
[youtube]http://www.youtube.com/watch?v=Mmi2lxRyMf4[/youtube]
So for now just let it heal. You're young, it'll heal fast. Benchpress or do pullups or something like that, just let it heal up. But, in the future, gain flexibility, and do some wide stanced stuff, ie, sumo deadlifts, slideboard work, and stretch the hip flexors out.
EDIT:
Wait, your lower back? Hmm, probably not hip flexor related. Anyway, don't squat and let it heal.