What Causes Spotting And Abdominal Pain Post Starting On Birth Control Pill?

I have noticed small stomach aches every now and then, just got on birth control after a week of not having it so I was originally on it for a few months, I had a baby 4 months ago, I'm not bleeding, and when I pee I have tiny blood clots. I spotted 5 hours ago for 20 minutes lightly normal red. What can this mean?