Help!

I had sex with my boyfriend tonight and after doing one particular position I experienced some discomfort because of the little bit of skin that is at the bottom of the opening on the vaginal hole. I got a mirror to take a look to check that there wasn't a rip or anything and noticed a little crevice. Is this normal?! I have only noticed it now and I've had gyno appointments in the past year so surely they would've told me if something didn't look right. Please help! Apparently it's called the posterior fourchette or something?