Hello, and welcome.
I'm sorry Roxy is showing some unusual symptoms that have you worried.
It sounds like she's doing well since the amputation, though! Cats are so adaptable! : )
Senior cats are more prone to develop medical conditions like hyperthyroidism, diabetes, liver and kidney issues and these can cause weight loss, in addition to other conditions. Because her sides are heaving, it sound like she's not taking in enough oxygen and is having difficulty breathing. Unfortunately, there's no home remedy for what you describe, so it's important to have Roxy seen by a vet, asap.
I strongly suggest that you bring her to a nearby ER vet center, now, so she can be evaluated in person and diagnosed, then, she can be treated.
With older cats, you really need to check everything out and catch anything that may be brewing, quickly, so it can be treated and she can feel better.
I hope all will be well with your Roxy and please let me know how she's doing and what the vet's findings are. Thanks!
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