Mild pain a couple days after intercourse?

I've noticed for a while that ever since my partner and I had sex one day in a position that cause deep penetration and slight discomfort or pain (I changed positions after I noticed it), that I've been having mild abdominal pain about two days after every time we have sex. This has happened multiple times if he penetrates deeply, but that deep penetration doesn't last long, as I tell him if I'm feeling discomfort. Does this sound like something bad, or could it be something simply like a bruised cervix? Is that something I should seek medical advice for if so? The pain only lasts a few days and only gets milder.

Additionally, I've been on birth control pills for about 3-4 months, if that has anything to do with it. (we aren't trying to conceive)