What Causes Abnormal Vaginal Bleeding With Stomach Cramps?

So I have been on the birth control pill for nearly 2 months and me and my boyfriend began being sexually active shortly after that, I m supposed to take the pill for 12 weeks without getting my period then get it, but I took it for 6 weeks to get it this week. So for 3 days I have been lightly spotting and today started bleeding bright red blood kind of heavily with some bad cramps. I just went to the bathroom and wiped blood clots (normal for me on my period) and a piece of gray mass out of myself. It was a few centimeters long and rectangular shaped and had little red lines in it. Is this my lining?