It can happen after sex, the vagina is extra blooded. So the chances of a vein popping is pretty big. I know it can be scary, but If isnt bright red with clot your fine. I have had it with my first pregnancy the whole first trimester.
I've had that happen on more than one location with different pregnancies.
It was kinda a dull red, quickly faded to pale pink
Probably just hit your cervix and that's what caused it. Was it bright red or pale?