I was hooking up with this guy and I was on my knees and he was standing when he came on my body. I tried to make sure no cum got on my vagina by putting my hand there but then a drop of cum got on the top of my vagina. I wiped it off right away and washed it too but I'm not on birth control and I'm worried I could get pregnant. What's the chances I could be?? Should I take the pill??