What should I do if I’m being accused of transmitting an STD?

1) If she accuses you of giving her a STD in a lawsuit, produce test results (and you may have to have your doctor testify also), that you are STD-free: if you don't have an STD, you cannot have given it to her. Evidence of no disease should be an absolute defense to the claim.
2) If you believe you are STD free, then when you file an answer to her lawsuit, file a counterclaim for frivolous litigation or malicious use of (legal) process, for bringing a lawsuit when there is no ground or reason for; seek what sanctions, including possibly legal fees, the court will give you for this.
3) If she is telling people that you gave her a STD when you are STD free, she is defaming you: you could also countersue for defamation, for the damage to your reputation.
4) If it's a civil lawsuit, she could sue you for monetary compensation for the effect on her life, which could be tens of thousands of dollars, but would have to prove that you were at least negligent, or careless, such as in having unprotected sex when you knew, or had reason to suspect, you had an STD but without telling her. Even if you had an STD, if you had no reason to know of it at the time, you should not be liable.
You could only be found liable criminally if 1) the authorities, not her, bring charges; and 2) there is evidence you essentially deliberately tried to infect her, or at least were "reckless" in exposing her--and reckless is behavior that goes beyond mere carelessness into not taking any account of the risk or harm to others. If that can't be shown, then again, even if you had a STD, you would not be liable.