Frequent UTI's since I was little?

When I as little I would get UTI's quite often, so often that when I was 2 I had to get a catheda inserted to make sure everything was okay and I have also had many ultrasounds. At sleepovers, if it was cold or I had a glass of soda before bed I would wake up with a UTI. It also happened more often when I was stressed. I stopped having UTI's for about 8 years and it started again recently when I became sexually active. I lost my virginity at 17, I am now 18 and have had six or so UTI's in the last year, I am wondering why I am so prone to UTI's, it's getting especially bad- in the last month I've had 3 more...and they seem to be getting much more painful. Does anyone know why I might be getting recurring symptoms and why this always happens to me? I have excellent hygiene and my GP's at a loss... Thanks