I think i have a water/bladder infection help!!?

Here is some background infomation that might help with answering this, im 16 years old, and the two times that i have had sex, i have got cystitis, probably because i didnt wee or get a shower straight after sex which i should of, but this time what i thought was cystitis is obviously not as i drank loads of cranberry juice and water, and also took some sachets of powder that help the cystitis go away, but this didnt work, i then told my mum, so she said that she would get my wee checked out as she works in a medical centre, when we get back from portugul, so its rougly been around 2-3 weeks that i have thought that i might have a water infection/bladder infection, i used to get them all the time when i was a child and finally got an operation for it. The symptons im getting is sometimes lower back pain, on the few days wanting to go to the toilet all the time, burning when i wee at the end, and sometimes before i even wee, i know i have had bladder/water infections when i was a kid, but im pretty scared, so this is why i need help, for anyone who might know what it is, or reausre me on what it might sound like? i am going to the doctors this week, but please help answer quick !!!!