I had a similar problem, the first time it wasn't diagnosed and I ended up in hospital on holiday, but my infections after that always followed sex. It was horrible. My GP prescribed prophylactic ABs, to take one after sex and that did the trick however I started getting the sensation of a UTI without infection so I saw a uro/gyny doctor and he prescribed topical estrogen. It does take a while to plump up the tissues, I used inside and out and I no longer get problems. I suppose mine were easier to target because they were triggered by one thing. I feel for you. I have a real thing about UTIs now especially after the hospitalisation as I was travelling alone so my GP gives me an emergency supply of ABs to give reassurance