What Causes Delays In Menstruation?

I had a surgical abortion on the 25.11.10 at 10 weeks, and experienced hardly any bleeding. Me and my boyfriend had protected sex 5 days after with the use of a condom. We continued to have protected sex, but on the condom broke one time, so i took the morning after pill on the 3rd day. There have been a couple of times where we have had unprotected sex, using the pull out method as i am now using a form of contraceptive, but my doctor did not tell me they take a month to work properly. I have not had a period for 7 weeks and it is only this week where i am now experiencing lower abdimonal pains, morning and night. I am also feeling very tired. Im hoping i am not pregnant again, i have taken 5 pregnancy tests and all have come out negative. Im not sure whether i have missed a period or not as it can take 4-8 weeks for my period to come back. Also, a couple of weeks ago i experienced spotting, which was a brownish colour.