What causes lethargy and loss of appetite in a 1+ old cat on a new diet for food allergies?, 9.0 out of 10 based on 1 rating

Two months ago my cat developed diarrhea and started vomiting. The vet took her off canned food and replaced her dry (Iams) with prescription foods. She didn’t like either flavor and now she’s on Science Diet. She is eating it but not with her old appetite. She doesn’t look as well groomed as usual, there’s a lack of animation to her face, she’s not active anymore, doesn’t greet us when we come home–all her old tricks appear gone. The vomiting stopped when we stopped the wet food. Her bowels appear pretty much back to normal. Why is she still not herself? She is an indoor cat, spayed, adopted from a shelter when she was approx. 6 mos. old. She doesn’t appear to have lost significant weight. She was always such a love and played with everyone. It is heartbreaking to see her like this.
EDIT: In answer to one of the responses, I did gradually mix in the food with the first two vet prescribed substitutes. When we got to the Science Diet, she ate a piece that accidently dropped on the floor so I went whole strength. At first, she was eating it but now it no longer appeals to her. Thank you for your responses.
EDIT II: FYI–Since adopting our cat, we had always given her Iams, both wet and dry, which I thought was the best commercial brand I could get. I did break down last night and give the girl some tuna and at least she seems to be speaking to me again. ;-) Thank you again for all your suggestions. I think I may give her some homemade food and see how that works.
EDIT III: I’ve made an appointment with another vet for tomorrow.
EDIT IV: Visit to the vet #2 today went very well–she was more responsive. Our cat was running a fever (105+ degrees) and her colon appeared ‘thickened’. The vet did not feel any blockage. She gave the cat a shot of penicillin and B-12. She also gave us a 10 day treatment of an antibiotic which the vet #1 had given us (but for only 5 days) which had worked very well. Once she went off the medication the symptoms came back and that’s when vet #1 diagnosed it as a food allergy. Vet #2 doesn’t seem to share that opinion. She thinks the original infection (vomiting and diarrhea) was only suppressed and when the medication was stopped it continued and has now developed into its present state. Our cat had lost close to 2 pounds so she recommended putting her back on her original food to get her to eat again. I gave it to her at dinner and she did eat. We are also running a complete set of bloodwork. We should have the results tomorrow. Taking her back tomorrow for a temp check.