She might be just stressed about using them and is tensing up unconsciously. I was probably at least 20 before I could comfortably use tampons on a regular basis, and I've actually had problems on occasion towards the end of my period where inserting a tampon completely is ridiculously painful (and not worth it).

Definitely get an appointment for her, regardless of the cause. I hope she can find a solution soon! It would suck for something like that to interfere with something she enjoys as much as swimming.