Ad hominem occurs when in the course of argument we don't defend our position or attack the opponent's arguments but attack the opponent with whom we are arguing. We state or imply that our opponent is not the right or an appropriate person to make the arguments they are using or perhaps even to discuss the subject. In the case you describe your opponent argued, she advanced considerations against your position, and you did the same against her. When all her arguments failed to persuade you or she couldn't find a good counter-argument, she then said that since you were impervious to her arguments, it was 'a waste of time to argue with a stupid person' such as you. She ceased to attack your argument, and attacked you instead.
I assume you are not stupid, and so abusing you was irrelevant to your arguments.
It is not the case, let me clarify, that to call someone 'stupid' as grounds for terminating an argument is always fallacious. It might be that someone really is stupid and in no position to argue a point, and it may be a perfectly legitimate (if hardly diplomatic or gentle) move to dismiss them as stupid or otherwise incompetent. Attacking the person may be the most effective way to avoid wasting time all round. I have concentrated on the circumstances you described, and in those circumstances I think ad hominen did occur and not legitimately.
My answer has been criticised on the grounds that 'A fallacy is always an argument or part of an argument (although it may be implicit/implied/understated). If somebody just calls you stupid, that's not necessarily their argument, it may just be them stating their opinion.'
I reply that argument did occur on both sides - indeed, extended argument. The termination of that argument on the opponent's side was an accusation of stupidity; unable to defeat the OP's arguments, or at least to persuade him that he was wrong, the opponent impeached his status as a competent opponent. This was not ad hominem ? Not an attack, and for the matter of that presumably unjustifiable, on the mental capacity of the opponent rather than on the opponent's arguments ?