What Causes Vaginal Bleeding With Abdominal Pain?

I am having mild to severe abdominal pain since two days ago. I am 2 wks late for period, thought I might be pregnant. Went to er last night and they took urine sample and came back negative for pregnancy. Two nights ago it started with a brown mucus discharge with little bright pink blood, light not heavy. Yesterday I bled all day but bright red, thin with cramping and abdominal pain. Today still the same, but about an hour ago I ve almost stopped bleeding, almost spotting.. Also little tissue has come out. What is going on?