With her age and history (of losing weight when left) I would just watch her. I like the kitten food idea, to try and encourage her to eat a bit more, add a bit of weight.

Is she a cat who normally plays with you? Maybe set a few play times, with a dangling toy. This will get her active which can increase her appetite.

I know taking them to the vet is also a stress for mot cats. But maybe you can take her in just for a weight? Then go back in 4 to 6 weeks to ensure she is putting it back on. At my vet, I take them in for weight during surgery hours -- the examine rooms are just sitting there empty. There is no charge. You could just call ahead and pick a time which works for them.