My girlfriend has burning around her after , what could be the problem?

I have had few ual partners, all of which have been tested and have had no STD's, and she has been tested recently and had no problems other than a UTI (urinary tract infection) which was taken care of. We do not use condoms so it can't be an allergic reaction to latex, and whether we use lubricant or not, it does not change that she is in pain afterwards. There has been only one instance where it did not hurt her afterwards but that was only once in a couple of months now. It did not happen when we first started having , it began to happen more and more often after we used a contraceptive (some type of spermicide), which is also when she got the UTI. We only used the spermicide once and never used it again since. She has tried AZO tablets, just to be sure it wasn't a UTI after she used the medicine the doctor prescribed, and it still didn't help. This has been going on for 3 months or so and has seriously started to deter us from having intimate relations.