Well, when was the last time she was weighed? Do you have a pre-move out weight? If so, I'd check to see if her weight has actually changed. If you don't have a pre-move out weight, weigh her as soon as you can so that you can moniter her to see if she loses more weight.
Is your dad pretty observant about the cats? Can he tell you if her behavior when you're not around has changed? Those things can be so subtle that if he's not really her behavior, then...he might not notice those little things. You know, is she sleeping more than before, grooming herself more, hiding, etc.
If her behavior hasn't really changed, then has the care she's been getting changed? Did you feed her more (including scraps) than your dad is feeding her? Does he play with her and keep her more active than you did? Those could be valid reasons for the weight loss.
If her behavior really hasn't changed, then I'd just keep a watchful eye on her, weighing her anytime you come over. If she loses more weight, then I'd bring her to the vet.
If her behavior has changed, I'd bring her in and make sure she's not got some underlying illness and if she checks out okay and they think she's stressed, you can use Feliway or Spirit essences to try to de-stress her.