As the OP is faced with a punter's ultimate dilemma, I'd say do nothing until certain. Despite the pressure you may feel you are under, sometimes doing nothing can be an option, as situations can be resolved in other ways.
There's no point in creating a volatile situation without just cause if she hasn't caught an STI. You learn from the lesson and move on.
If she has any STI symptoms, you will know in a day or two, and sorry to say, but the scapel will be out. If no symptoms, you need to abstain from sex without condoms with her until you are confirmed as clear. Feign back ache or something, but the key factors are to act as normal as possible so as not to attract attention. Dont do anything in her presence that might be construed as different or unusual behaviour. Women start to ask questions, and you soon get caught out.
Dealing with the inevitable fallout if she has (and you have) an STI will be immensely difficult. You will be asked some very searching questions, and having been wrongly accused of philandering in the past over, you have my sympathy. A woman scorned is not a pleasant sight, and it seriously buggers up all future trust. If you stick together, you feel you are on probation, with your every move, text and email being questioned. I've been there, and twice been a gnat's cock away from being booted out - and not for activities with WGs.
Best of luck.