Could be a number of things - you might not be adequately lubricated so friction causes pain and may result in tearing - particularly if your partner is well endowered or you might even have an internal benign cyst that is getting agitated
nip along to a woman's clinic and get checked out - if you have only had sex 3 times your vagina is still getting used to it so don't over worry - it is not always so fabulous in the early days
Yeah its probably that it's not well "lubricated" down there. I'm 25 and it happens to me every once in a while too. You could buy lube or even use spit.
I have never hurd of this..
But maybe talk to a doctor or just not have sex in tell the right person comes alone and u marry that person.