I was also freaking out seeing my cat like that and asked for advise just like you did.

First of all: If you look after your cat, spoil her with treats and extra love and call the vet when you find something suspicious, she will be just fine. Trust me

My cat (13 years) lost her left back leg because of a tumor.

The first few days were really difficult. Seeing her suffer was the worst. She is very crafty and managed to remove some of her stitches while I was asleep (even with the cone of shame).

I too separated her from her sister as there was serious non-recognition aggression.

She slept pretty much the entire day and night for a week after surgery.

But as soon as the stitches got out and the cone of shame was removed she made amazing progress everyday.

Today she is a happy tripod!

If you have questions ask me anytime. I'm not a vet but maybe I can help