UTI's

I'm 17 years old & keep getting symptoms of UTIs. I lost my virginity to my current boyfriend at 16 and made the mistake of not peeing after sex so of course I got a UTI but I went to the doctor and got medicine. well after a couple of months and other days of sex I got the pain again so I got more medicine. and it kept happening but I didn't wanna go to the doctor because my mom would start to question. so here I am 17 and still have the symptoms like a UTI. but I wanna know why I keep getting them. I'm doing everything right & keeping things clean.