What Causes Stomach Cramps And Chunks In Menstrual Flow?

I use Nuvaring birth control. In january I got off my cycle for awhile but my boyfriend and I used condoms. We regularly use condoms when I'm on my birth control just for "cleanliness" reasons. I was supposed to put the ring back in on the 7th of january but i ended up not putting it back in til the 20th which threw off my cycle. I was supposed to get my period on the 20th and I ended up getting on the 9th of february (which is correct with the new cycle). my boyfriend was concerned that I should have waited til my period began and put the ring in on the final day, like I did when I first started. But I'm scared because this is my 2nd day of my period and I had a very bad headache and very painful cramps. When I pulled my tampon out there was two big chunks (i've never had chunks before) of bloody tissue. I don't know what they are...I'm so scared it was some sort of fetus or that I'm shedding too much of my uterus. I'm 18 and have been on birth control since July 2009