Cat not eating enough

Got a 2 year rescue from a shelter about 2 weeks ago. She was a surrender from a woman who had her since a kitten. She is happy, playful and healthy and ate Whiskas according to their paper work. She is 8.3 pounds..according to the Vet she is rated PERFECT/IDEAL as for body size/shape/eight. 

We got her home and she was eating a good 4-5 oz a day not problem of quality, grain free wet food. Then she got a upper respiratory and on zithromax for 5 days. Her appetite gradually has decreased. She finished her 5 day dose last Thursday but is eating maybe 1.5-2 oz a day. 

She is peeing, pooping, playing and well hydrated and no vomiting or anything. But won't eat more than that.

Took her back to the vet a week early just out of fear (we lost our 10 yr old to heart failure a month ago). They checked her over and said the respiratory was doing fine all was progressing. They chalk up her hiding and not eating much to maybe still stress of a new place and being given oral meds. 

I have tried putting out 3 or 4 kinds of wet food at a time spaced around. NOPE. I even put out maybe an oz of dry and she nibbles at that (despite never eating except her week at the shelter) but will not eat more than maybe an ounce or so of wet. 

Not sure what to do? Just seems like she isn't eating enough to maintain...

As I was reading through your post and saw shelter I thought to myself, and she doesn't have a uri?! haha

Sorry but I pull from the shelter all the time and they are always either sick then, or within the first week it hits them.

Anyway, just a thought, but try Fancy Feast canned. Chicken pate and the fish variety, but classics, which are all pates.

She is def. not eating enough daily, 1.5-2 oz is hardly anything. She also needs to stay wrll hydrated.

If the FF doesn't do the trick, pick up some chicken only baby food, warm it up a bit, she will likely go to town and eat all that up.

She might need recharging of her appetite, and the baby food will do the job.

To address possible stress, you could purchase a couple Feliway diffusers for your home.

My cats used to like Whiskas wet food, but now they just don't anymore.    I agree with catwoman707 and would try Fancy Feast Classics. My cats also really like the Purina Pro Plan Classic varieties (make sure it says Classic and not Ground).