It can be really normal for some women to bleed after sex. Your cervix is more likely to bleed after sex when pregnant and irritation can cause it. I'm sure there are a lot of other harmless reasons too. Unless you have cramping and back ache and other symptoms then it's most likely just an irritation that you shouldn't worry about.
Just pop a pad on and keep an eye on it. If you're concerned then definitely call your midwife or GP, no point in worrying when you don't have to.
Take care x
I have had it in this pregnancy and the last, was worse in my last pregnancy was on and off for weeeks. Turned out just to be my cervix. I was getting it even if I didnt have sex.Even though it is a common thing to happen it is still scary though.