My periods are really irregular

Hi im a 13 year old girl and my periods have been really irregular. They have been 20 days late these past 4 months but I eventually do get my period after this long period of time. I've never had sex with a human being before, but I masturbate frequently. Im pretty sure masturbation doesn't really affect pregnancy but im not completely sure now. The weird thing is I had a Urinary Tract Infection those 4 months ago and had to take antibiotics but im not sure if they are still messing with me for I dont take them anymore. I would really like some help because im going to the doctor and dont wana freak out my mom with some big weird pregnancy