What Causes Bright Red Watery Blood During Periods?

hi, i am having bright and i mean bright red very watery blood. it is the fourth day of my period and i thought i was almost done... just a little spotting on my third day. so i did not wear any protection. i hav not had a problem all day untill about 2:30 pm and it started to feel like i was making a mess so i went to the bathroom and BRIGHT red watery blood streamed out as if it were urin. what could this be?