Pain during sex?

My husband and I have been together since high school and I lost my virginity to him. Often, when we have sex, there’s a sharp stabbing pain... it’s not constant and it doesn’t happen every time. But more than 50% of the time I’d say. It happens when he goes really deep and we just thought it was him hitting my cervix to cause the pain. But he doesn’t have a LARGE penis. I’d say it’s average size. Maybe I have a short vagina? Now that we’re TTC, I’m concerned that there might be something wrong up there. It didn’t happen when we were in high school... come to think of it, when I’m really turned on or horny, it doesn’t happen at all. Weird. What could this be?!