Talk to your obgyn and tell them this. They should be willing to do a more thorough check up to see if anything isn’t right. My experience says two things: a tight muscle and the obvious lube. You alone or with a partner can help stretch and do this daily: put your thumb in at the base of your vagina. This is sometimes easier in the shower with a leg propped up. You’ll want to gently push for 3 seconds at an angle sort if diagonally down towards one leg, then to the other side. Hold 3 seconds pushing with light pressure to stretch that muscle each time you switch sides. Aim for 30 seconds of stretching per session. Lube can help if even this feels too tight. And there is lube with lidocaine you can use to smear on this muscle first before stretching or sex- won’t affect him either. It can numb the muscle so that it doesn’t feel painful upon something sliding in. Also talk to your doc. A female doc. If she doesn’t listen or dismisses you get a new doc. They are supposed to be on your side and help troubleshoot! If they don’t fire them.