Is this normal?

I have endometriosis and i started taking birth control pills to help it. The pills were working very well for 5 months, but my last period was really awful. It seemed like the pain was even worse than before i started taking birth control pills, and my period lasted much longer than it ever had before. I have been taking my pills on time, 
so I don't know why this last period was the worst one i've ever experienced. Has this ever happened to anyone else?