What Might Cause Delay In Periods After An Abortion?

Dear Doctor.I am 29 and I have a son who is 1year 9-months. I recently got pregnant, and I did abortion on Sept 08, 2011. On Nov 06, I got my periods after the the abortion (66-days after the abortion). I was supposed to get my periods on Dec 04 (29-days), but upto now I have not get my periods. Now it 54-days now.

After the abortion, I didn't have sex, until Dec 18, which I used a condom. I am sacred. Do periods get late after having a abortion ?? Pls tell me.

I did a HCG test Dec 27and became Negative. Was it worth taking this HCG test yesterday as I had sex on Dec 18??