What Could Cause Menstrual Irregularity?

I just started birth control a month ago & am on the pill. I was supposed to get my period for only 4 days (about the same amount of time i normally get it) but i got it maybe 2 or 3 days before i was supposed to & it started off light. The last day i was supposed to be on my period which was Wednesday but instead, i had severe abdominal pains to the point where I could barely move. I then used the restroom &changed my tampon but found i was bleeding very heavilly which i wasn't supposed to. Not only that but on my tampon was a white piece of flesh looking meat. I know it wasn't a blood clot because i usually get them while menstruating. This freaked me out. after the "thing" was out my tummy felt kind of better but maybe 2 hours later it happened again. this all happened while i was working so i could not leave work. The next dayI called my clinic where i got my birth control prescribed & they told me to go in to take a pregnancy test.The test was negative & the only answer i got was that it could've been mucus of some kind but I truly am concerned because this has never happened to me. I am 19 btw. I have never been pregnant.