Accusations

Now I haven't responded to this but my narc had text saying that he knew what I had done and he was really angry and upset.
He is accusing me of doing some really degrading sexual thing with an unnamed person.

Now I'm plagued with guilt even tho I haven't done what he said?
The fact is i kissed someone a couple of years ago I'm a drunken mistake, and felt awful about it for such a long time. I never told him obviously and it never happened again.

I have a feeling the n is either completely making the whole thing up or he has found out about the kiss and turned into something really dirty.

Why do I feel so guilty and like I've done something so awful? I feel like the absolute lowest of the low even tho I would never do what he said I did? I feel plagued with guilt now because of that kiss. Why has he got to me so much ? After everything he's done to me I still feel like the guilty party.