What Causes Blood Blister In Vagina After Having Sex?

Hi, im an 19 year old girl and earlier on a huge gush of blood came from down below while i was having sex with my partner, then a while later i noticed i have what looks like a blood blister on the inside of the flappy bits of my vagina (i dont know what they're called) and is also extremely dry and sore. I am very worried. Please help.