What Causes Gush Of Watery Red Blood From Vagina?

Hi. My period wasnt due for 4-5 days bit 5 days before i got light pink discharge a couple times before. Then 4 days ago in the middle of the day i sat on the toilet and a gush of very thin watery bright red blood with no clots and slight lower back pain. Pain os coming and going, but blood is still really thin and watery with no clots and also comes and goes, im on my second day of this? Having hot flushes and sweats, pale in the face and extremely tired.