I popped a blood vessel behind my knee after doing a leg curl. what should i do?

Ouch!: I'm guessing you probably have a bruise, and you may be a bit sore. But, overall, your exercise is probably doing you far more good than harm! you may want to hold off on the leg curls until you're less sore (if it hurts), or try a different exercise to work your hamstrings. Btw, it might be time to mix it up a little and do something different: swimming or a spin class, for example. Good luck.