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.
It seems to be a common problem. Many of my friends have also complained about irregular periods after tubal ligation. One cause might be post tubal ligation syndrome...if not, there might be an underlying medical condition. It is best to consult your gynaecologist on why this is happening though...
Like what anonymous said, I'd say you should consult your gynae too! I hadn't had any ligation done, but have heard of people who did get pregnant even with it done.