I too was constantly getting thrush, at least once a month before TTC. It got so bad that the Canisten cream just wasn't working properly, so was given a strong script (however you cant take it when TTC) which killed it off quick smart. It also occasionally caused bladder infections too. I was told too that nylon nickers or stockings can bring it on.

Then a few months later it came back, tried canestan & it wasnt doin anything, got to Dr's, they sent me for swabs (however can't have used canestan for at least 48hrs before), which came back negative for thrush. She said it was most likely a skin iritation like sirisus or dermatitus & told me to stop using soap down there. Started using 'Dove', never had it since.