Cheating on someone is a shitty move and I personally think you are totally in your rights to act ‘aloof’ or weary for enough time to process what has happened.

Maybe they are acting super nice because they know they did something fucked up and now over compensate to try and to regain some normalcy in their everyday life, but I’d say that normalcy should arrive when you yourself decide if that is a path you are ok with and in the mean time you are allowed whatever feelings you desire.

But that’s just my view on it.