Sexually active.....

So I've been with this guy for about 8 months and for about the past 3 months we have been having sex. I'm on the pill, we were using condoms at first and now not so much.. he's the only person I've ever been sexually active with. I'm pretty sure I have a UTI at the moment I told my mom I needed to make a doctors apt and of course she asked why and I told her but she doesn't know I'm having sex nor does my doctor. How important is it that my doctor knows when i go in for the uti that I'm having sex?

Edit: I'm 16.