I have unprotected sex w/ BF. I had an extremely light period for 2 days. It's now 10 days later and I'm having light brown discharge w/back pain. Why?

Pregnant?? you must always rule out pregnant. What concerns me most is an ectopic pregnancy since you have back pain. Please seek evaluation from your gyn to rule out pregnancy and an ectopic. otherwise you are just lucky. take responsibility and use contraception if you don't want to be pregnant.