Why Is There Only A Few Drops Of Blood During Menstruation?

I have questions about my periods. My last period was on the 2/15/11 (1st day), and before that was 12/29/10 (1st day). On 3/26/11, I only has a few drops of blood and it went clean afterward. Only had intercourse once on 2/27/11. I wondering if I am pregnant, or there is something wrong with me. Why my period only a few drops. never happen before.