Dont worry its not really an STI you probably did get it from having sex but you can get it at anytime sex or not it is more common in woman though.if this is thrush you can walk into any pharmacy and get some medication for it (Its ussually cream) and it works straight away! hurray if thrush cream doesnt work then seek medical advice as it could be an STI but what you are describing does sound just like thrush good luck