TMI problem

Recently, every time I have sex, I get thrush symptoms a few days later. It's very frustrating as it's getting quite painful and itchy. I've been going to pharmicies to get Canesten pessaries and cream and these work. Then I have sex and it's back. I gave my boyfriend the single dose fluxanole tablet the last time it happened and despite that, I have it again now.

I've been with my boyfriend for nearly five years and this never used to happen. I'm not on contraception and we don't use condoms. I work full time quite far away from home and can't get time off to see a doctor so was wondering whether to visit a GUM clinic some evening? I doubt it's an STI as I have only had one sexual partner and he has been clean since before we starting going out. Would it be worthwhile to go to a GUM clinic?