Ok I'm going to name the dudes to make it easier. the guy (20M) you've been seeing for a while, we're going to call him Mike.

The guy you kissed at the bar, we're going to call him Bob.

Anyways, to vastly oversimplify and create a false dichotomy, I'm seeing 2 main scenarios:

Scenario A) Mike is nice and all, but you aren't ready to commit to Mike; maybe you were ready, but not as much after the little rejection, even less so when drunk.

Scenario B) Bob took advantage of the alcohol at the bar and kissed you without your consent.

I don't think there's easy answers. Maybe just ask Mike to be your "It's complicated". Although ideally you'd want to not keep information from Mike, people are not logical automata who just accept information like that.
If Mike is open minded enough, I would tell him, as keeping secrets can be costly.
Or maybe you might be able to frame it with the exclusivity thing. "I know we're not dating, and I went to the bar a couple days ago. Somebody kissed me and in the moment I brushed it off but now I'm worried I should have tried harder to be exclusive to you even though we're not dating yet."

If there is a chance the rumor of the kiss with Bob to could spread to Mike, do yourself a favor and call Mike as soon as you possibly can. Like the last thing you want is for somebody else to be telling Mike about that kiss.

I don't know what I would do. You gotta play this by ear. Good luck!