Implantation bleeding?

I have always had irregular periods. There doesn't seem to be any reason for it, as far as the doctors can see - I just don't always ovulate, and no one is sure why. When I do have my periods they are typically very painful. I was surprised to find out that I was pregnant about 3 years ago, and the doctors told me that typically women who have unexplained irregularity in their menstrual cycles usually find that after having a baby things clear up and work as they are supposed to. For a little while, that seemed to be the case - I had a period every month for several months, and they weren't painful the way they had been before. However, the irregularity started up again and things went back to the way it had been.

Currently, my last period started on 12/26/09. So it's been approximately 4 months - nothing unusual. I'm not on any form of birth control. My husband and I aren't necessarily trying NOT to get pregnant, but we're not trying to either. Mostly when we have sex, he just pulls out prior to ejaculation. Friday night (4/23/10) we had sex, and he ejaculated inside of me.

Starting Sunday (4/25/10) I started having some very light pinkish spotting. I expected to be finally having a period, except the rather than getting heavier, the spotting stopped today. The information I have seen online indicates that implantation typically takes place 6-12 days after fertilization, but I have no idea when I last ovulated. This is exactly what happened when I got pregnant with my daughter. I hadn't had a period since December and then in April, had unprotected sex with my husband which resulted in very light implantation bleeding a few days later. If I am pregnant, I think it's too soon for any symptoms to be showing up, although I am unusually fatigued today. What do you think the chances are that I'm pregnant?