Firstly, taking birth control while pregnant does not harm the baby. My Doctor told me that when I called to set up a test. I was curious, but he told me to continue taking it. For future reference if you are not pregnant
When I was on the pill, I got really light periods. Much like the one you described having last. As for the light bleeding you had a few weeks after you got off the pill, that was also normal. It's hard to explain. But, after you have already you your period on the pill ( the light spotting) after you get off the pill a few weeks later you will have light spotting. I used to do it all the time. ( I had to get on and off my pill alot, because my Doctors were trying to find out what was making me so sick...turns out it was the pill
There is a chance you could be pregnant. When was the last time you had had sex?