Are they denying something you both know was said or done?
Behaviour like this is sometimes called
Denying what was plainly said or done, persistently enough that the other person starts doubting their own recollection.
Deny the behaviour, attack the person raising it, and recast yourself as the real victim of their raising it.
What would tell them apart
Denial on its own, repeated until you start doubting your own memory of what happened, is one pattern. A second pattern adds two more steps on top of a denial like that: turning around to attack the person who raised it, and then presenting themselves as the one who has been wronged by the accusation.
Why it might be none of these
Two people can genuinely remember an exchange differently — that's ordinary, and extremely common. What points toward something more deliberate is whether the denial only runs one way and is aimed specifically at shaking your confidence in your own judgement, rather than at working out what actually happened.
None of this covers stalking, doxxing, swatting or harassment campaigns. If any of that is closer to what you're facing, go straight to Red lines instead — the advice on this page was not written for it.