Yes. You should be concerned with both.
Under normal circumstances, terming an employee who is not meeting production standards is legal (not wrongful) regardless of whether she is within her first 90 days or not. In every state except Montana, and even sometimes in Montana, there is no significance whatsoever to "the first 90 days" or 60 days, or 30 days, or any other number of days, when it comes to the legality of termination. (It might have a significance regarding your liability for unemployment, but that's a totally different issue.) But there is nothing illegal, again under normal circumstances, about terming an employee for performance issues unless a legally binding and enforceable contract says that you cannot. In which case what you have is a contract breach and not a wrongful term.
However, you didn't ask if you wrongfully termed her; you asked if you should be concerned about a wrongful termination lawsuit, and in my opinion you should. Having sex, consensual or not, with an employee is the kind of stupid that gives stupid a bad name. That is unforgivably poor judgment. Then you crossed the line into criminally stupid when you proceeded to fire her.
I have no reason to disbelieve you when you say that first, she was not meeting production standards or second, that the sex was consensual. But she'd have to be just as lacking in sense and judgment as you are not to at least consider suing for wrongful term, sexual harassment or both, under the circumstances you've described. And if she follows up on it, all I can say is that you'd better have independent documentation of her performance AND a tape recording of her making it clear that the sex was consensual if you expect to survive a claim, because whether you are guilty or not, you couldn't have done a better job of making yourself LOOK guilty if you'd been TRYING to get sued.
