It's okay, I guess. $hit happens. Just take it in your stride. Go speak to her about this, laugh it out and forget about it.
Two issues here. One is today's environment is one where workplace sex isn't as innocent as it once was to some people, but so far nothing has happened. I'm assuming that's what's upsetting you in part. The second issue is you're drawing a lot of conclusions from this that don't really have anything to do with this unless you're planning on sleeping with other employees after saying this one was a mistake, because there's nothing these people are going to say that's going to have anything to do with who you're going to date in the future away from work. The second issue is an anxiety issue, the first a legitimate concern. You have to analyze it that way, separating out what you can learn from and what is irrational thinking, which since you're posting on an anxiety forum we have to assume you think you have an anxiety problem. If this is a pattern for you, you might want to see a therapist and make your life easier by being easier on yourself. If this is an exception then that doesn't indicate an anxiety problem. People get attracted to one another, that's life. The only complication here is who you did it with. If nothing is said about it, you both had a good time and it's over. But you're right, in this day and age, don't go there. It's kind of sad in a way, because that's how most people used to find their mates, at work.
Time heals? I really think in this case it will. If she talks . . . she makes herself look bad if she makes you look bad. And in this day and age, hook ups are not uncommon. I'm not going to minimize your feelings though. I'm just saying this should dull with time and just stay busy and focus in on WORK when at work. good luck