What Causes Cramping, Nausea And Passing Of Lumpy Discharge?

I was 4 days over due my period which I got this morning, within an hour I had extreme cramping which felt like contractions coming every few minutes I was doubled over and couldn't stay still. Painkillers eased the pain. This afternoon I have had a fever feel Very nauseaous and passed what looks like a lump of flesh. It is the size of my thumb and shaped like a sac. I didn't feel pregnant(im a mother already) dont think this is a miscarriage, could something be seriously wrong?