You may be dealing with vaginismus, which is a condition where the vaginal muscles involuntarily tense up in anticipation of insertion. It can make sex (or any other kinds of insertion, like tampons or pelvic exams) extremely painful, because you’re stretching the tensed muscles the same way a muscle cramp would.

It’s largely considered a psychological condition, because it is typically based on anxiety surrounding the insertion. And that anxiety can quickly become self-perpetuating, as pain from insertion will likely cause future anxiety. Therapy (and practice) can help alleviate that anxiety, which can alleviate the tensed muscles, which alleviates the pain.