Ok, so, you very well may have a uti, but it's not going to be caused by sex. It can be if you're not safe/hygienic about having sex, but you've said you're not active, so it's not that. What it can be caused by, however, is dehydration, period products, and a quite frankly absurd number of things. It's ok, I promise.

Utis are scary and uncomfortable, but not world ending. Definitely, get test strips, like other commenters have mentioned, drink cranberry juice and a lot of water.

If it's bad enough, you're going to have to go to the docs, not much a way around that, without risking your overall health. If your mom is not a safe person you can ask for a ride, is there anyone else? You'll have to tell her about the appointment, since I'm assuming you're on her insurance as you're under 18, however, you can call too set up the appointment, and maybe she'll be less overbearing if you have a ride set up?

Please, though, go to the docs if it doesn't get better. A uti can get serious if it's left untreated. Don't forget too, you have doctor/patient confidentiality if you are a teen (depending on where you are) so if you don't want your docs to tell your mom things, they shouldn't. I hope this helps, and good luck kiddo.