i suppose if you are dry this could mean your urethral opening is getting irritated more easily. this could be a factor.
i used to get urinary infections all the time too. it was a pain in the ass. the thing that seems to have made it better is going to the toilet after sex. you are supposed to do this to flush out infections.
at the time my boyfriend was living with his parents, so to get to the bathroom i had to go down a ladder, across his hall past his parents. lol. that wasnt happening! since he's moved house it's much easier to get to a toilet, i've been peeing straight after sex and it seems to have stopped the problem.
cranberry juice is supposed to help prevent cystitis by stopping the infection sticking to your bladder wall.... you can also get cranberry tablets which i have been taking daily, i think this has helped.
have you tried these things? it's still happening? well there is a condition women can get called intersitial cystitis or something. doctor's often misdiagnose it as a simple urinary track infection. this is something worth investigating. ask your doctor