What is a miscarriage

So my period has been late for a while (1 month) and all of a sudden my period has gone while and I’m bleeding like crazy. It overfills my cup and I have stained all of my jeans. I have the worst cramps of my life.

I am sexually active and on birth control but I have missed a couple of pills. I was worried about maybe being pregnant but then I started my period or whatever this is. Can someone help me? Could this be a miscarriage or am I just going insane!?