Extremely bad breakthrough bleeding cramps!!?

I am experiencing very bad cramps that almost unbearable. I am bleeding very heavy and should not be on my period for another week. I've had very sore for two weeks. I am on birth control but have been taking it for a year and always had a regular period. The pain is so bad I can barely stay at school or hold myself back from crying. There are clumpy chunks of uterine lining (I think that's what it is) coming out. I've talked to my mom about it but she doesn't seem the slightest concerned but I know something is not right. Is there anything I can do to relieve some of the pain? Has anyone experienced this before? Please help! Thanks so much in advanced.