I agree with the above poster. AND with your doctor telling you about your uterus - just remember that you CAN get pregnant b/c you are still ovulating normally etc, it's just that you probably won't be able to carry a baby to full term (meaning you would suffer a miscarriage). I would start using protection :-)
Also, brown spotting is usually old blood. This used to happen to me all the time. It was actually really disgusting b/c it would be a dark red then gradually go to a blackish color. Like the other poster said, it could most definitely be from your birth control. If it keeps happening every month, talk to your doctor about possibly switching your pill or if she's concerned about it at all. Good Luck!!
The spotting could be from a number of things. If you are worried about being pregnant then just wait a bit and take a test. If it comes back negative you can always wait and see how your period is. If it's lighter or shorter than normal take another test or go to your gyn for a blood test.
The spotting could just be break through bleeding from the pill you are on since you are almost to the sugar pills. That happens to me a lot. If it continues maybe talk to your doc about it and if anything they can try a different BC. Good luck hun and keep me posted!