So, I started birth control almost a month ago. I've had protected sex with my boyfriend, pill and condom. Except for a couple times where he would put it in for a while before putting on a condom. (Before he would finish)

Well my period wasn't supposed to start til tomorrow. But i got it yesterday and I've had really bad cramps, and really tender breasts. These are common for me when I get my period but this time they were a little worse. Anyways, I went to change my tampon a minute ago and this came out when I peed. Anyone know what this is?!? Just membrane or what? Never had anything like this come out before