Weight Loss 6 year old female is losing weight vet can't find anything

I just joined today looking for more I can do for my 6 year old female guinea pig, Plushie. In the last couple months I noticed she was starting to slim down. In the last couple weeks I noticed it had gotten to the point it was severe. I have severe guinea pig allergies so I can't hold my pigs regularly without protective wear. So I got her a vet appointment right away because I noticed she had also lost her voice when trying to wheek really loud and would cough occasionally. She doesn't have running nose or eyes and her breathing isn't labored or whistling. With the weight loss and coughing though I thought respiratory so I made a vet appointment with a exotics vet and they were able to get me in the next day. That was Tuesday 6/26. He listened to her breathing, did x-rays and a blood test. He heard my concerns it was a respiratory but after not hearing it he looked for diabetes with the blood test and got her an x-ray for stones or cancer. She was down to 663g but the blood and x-rays came back negative. He even said she had a full belly of food and that her poop looked alright even though she have much in her crate despite being in it for a 2 hour long car ride. He gave me the option of collecting a urine sample for her next appointment to check for diabetes again even though he doesn't think it's that because it's so rare in guinea pigs. At the end of it he said he didn't know what was wrong but agreed I should definitely be worried as she had lost a ton of weight since her last appointment in February for another matter. I can't remember exactly how much she weighed back then. He gave me a 10 day prescription for baytril which she has been eating like a champ. As for other options he gave me it was hat she might just not be absorbing the nutrients and to switch her to a smaller grain food. I've also inquired if it would be worth switching her to the elder care food given her age and he seemed to think it wouldn't hurt and to also log her weight once a week.

Since then she has lost another 30g and her voice is still gone four days into the antibiotics. I've started giving her a serving of critical care and put her in isolation to monitor her food intake as well as her droppings. She isn't acting strange. She's just as bright eyed, active and ravenous as the day I got her in 2016. She's been shedding a lot recently but hasn't developed thin fur or balding patches. She's a rex so it can be hard to tell if her coat is glossy but she feels very soft as always. Despite her being 6 years old my vet said she should live closer to ten years than eight. Today when I weighed her to find she had lost even more weight I felt so frustrated to the point of tears that she's still just wasting away. Right now I want to get her through her antibiotics and just want to see her get some weight back on before scheduling her next appointment. Is there anything else I can give her to help her pack on the grams again? She's already on oxbow feed, I can't remember what kind exactly, timothy hay, fresh grass, fresh veggies and fruit treats. I also have a vitamin c and bone supplement she gets once a day.

As for additional information before I can forget, the reason she went in February was for a check up after two of her herd mates died suddenly within a couple weeks of each other. They ran a few tests and called us back after her stool test to give us a clean bill of health. She was fine after that until just recently. As for her herd mates I never found out what happened. They weren't showing signs of being sick, no running nose or eyes, labored breathing, weight loss, etc... The older of the two had a couple bald spots I was getting ready to call in for an appointment when she just died a few days later. The second one then died with absolutely no warning a couple weeks after that. I freaked out at that point and got Plushie, the last remaining of her herd, in as fast as I could. The vet thinks it was probably some kind of heart failure but as I live several hours from the nearest place I could have gotten a necropsy I'll never know. Also about six months ago I switched from feeding them half Higgens Sunburst and half Kayatee Forti Diet to what they're on now. She doesn't seem to like it as much but she still eats readily. She's also not alone as after the passing of the other two I reintroduced two females I had separated several months prior due to one of the girls that died not getting along.

I'm looking for more advice to keep her going as I wait out the antibiotics. It feels so frustrating watching her waste away and not knowing what's going on.