Are you sure its related to the sex thing? Have you changed anything else: your type of tampon, your type of underwear? These are just as common triggers as sex. My youngest was getting them chronically at 13 (and yes, we certain she wasn't active) and it turned out to be both her underwear, and the type of tampon she used, as well as how long the tampon was in.

PS: I surely hope you're on the pill AND using condoms. Cameron is living proof how easy it is to get pregnant at 17.