Hi and welcome to YT! Georgie did the same thing last year when went on vacation. He stayed with his Grandpa who he stays with during the day when we are at work. I think it might take time to get them back to their normal routine. If she is eating and drinking normal then I would just give her time. If your Dad lives close take her for a visit
As a side note my nieces Yorkie has seperation anxiety so bad that she can't even show him her suitcase or he gets upset. I don't think your Mitsi has an issue like that but maybe needs a little reassurance that you aren't going anywhere.