6days late and bleeding heavily

So today would have been my 6th day late on AF.. took a couple pregnancy tests during the week and all negative.

All of a sudden I've started bleeding today, but it's extremely heavy like nothing I've had before and VERY painful on my lower abdomen. I'm having the change my tampons every 2 hrs or less as the bloods going through.

Anyone been through anything similar?

My period are always quite normal and never this heavy or painful, it's borderlining pain that I want to cry.