A pregnancy after a tubal ligation is a dangerous one. An ectopic pregnancy can cost your life if not attended to early.
I just had ligation done early April, and haven't had my period. I'll see my surgeon next week, and will update!
Update:
The first few months after the ligation, there may be some changes to your period. After it stabilises in a few months, there should be no worries about unprotected sex.