Positive test, but not pregnant?

I tested positive two days ago and was, of course, absolutely thrilled because my partner and I have been trying to conceive for quite a while. I tested two days after I was overdue. But now, two days later, I suddenly got my period? The bleeding is lighter than usual, but I still need two pads.. My question now, do you know how to tell the difference between a real period and the monthly bleeding that can happen during pregnancy? (I didn't even know this was possible at first, but I read about it; some women have a pseudo-period every month. It's just not very common) Thanks in advance!