Vibrator Questions

I'm feeling fairly lost here. I'm contemplating getting a vibrator but I room (same room) with a very religious girl and so I'd have to use it only at my boyfriends apartment. But then I feel I'd have to tell him about it and make him feel threatened that I need it because we don't have sex often enough. 
I mean to be honest we don't, or I wouldn't need the vibrator to make my periods less painful but how do you tell your guy that nicely?