PLEASE HELP NEVER HAPPEND BEFORE

I have been on prenatal for about 5 months now and never had a bad or painful period. But I got my period 3 days early worst pain ever and really really heavy bleeding and a clot. I was really dissy and light headed all day never felt this way before. But I today wan my day to get my period and I am still bleeding but barley and no cramps at all or pain feel great. Please help I don't know what this means