I didn't have my period for 4 month, I started taking meds so I could have it. My boyfriend and I had sex on my period while I was 4 days in to taking birth control. I want to know if I could possibly gotten pregnant, from the eggs already in my uterus those 4 months?

Of course. Any time you have sex, even with protection, pregnancy is possible.

Yes, many women get pregnant on their period