So you can check for an ACL injury beyond just listening for clicking. They should have done a physical exam including a cranial draw test. You can literally feel an unstable knee. 

Overall that would not have changed the need for surgery of cancer was involved. Expedient amputation is the best course of action in most circumstances. Even in the worst cases it relieves pain and distress that comes with the latter stages of osteosarcoma. You did the right thing. 

So the two day thing sounds about right. Increased pain and discomfort are pretty common. Your pup was likely on some good pain meds including a dog safe NSAID. A side effect being their general arthritis was suppressed. Coming off a NSAID will lead to a temporary increase in symptoms beyond baseline plus the discomfort from the surgery and adapting to the added weight. 

For amputation don't make any large judgment calls until your at least a month out. Dogs improve rapidly over a month with good care. 

Let's work on the symptoms now, first thing to do is call back the surgeon. See if they have any thoughts and may be able to adjust the medication to continue with an NSAID or other arthritis treatment. 

Look into a cruicet brace for their back legs. Even if their ACL is going or gone this can help support their knees whole their stamina recovers from surgery before measures like an ACL repair may be possible. 

Thirdly icing and heat, both gentle heat and icing can help reduce inflammation at the wound site and in their joints. This can help relieve symptoms. 

Fourth as soon as that incision is closed up get them in the water. Look into hydrotherapy or a way for them to swim. Get them a life jacket so they can float when they get tired. Swimming is a great way to build stamina while unloading joints and relieving pain. 

Fifth look into amd ask the vet about range of motion exercises. These are usually just gently moving your dog's joints through their range of motion to help them stretch and stay limber. 

Our nine year old front amp tripawd has had three major leg surgeries: his amputation due to unknown trauma the day before we brought him home at five months old; then an FHO on his hip on the same side about six months later; then about two years ago he had TPLO surgery on his other knee for an ACL year. I won’t lie - the recovery can be brutal but he has had such a resilient and fighting spirit and bounced back with good support and active pain management. Everything above sounds right to me. You can do this.

I would wait at least a month, my dog has luxating patellas and it’s taken him two months to really handle walking on his remaining hind leg. He is small breed and is taking steroids as part of his chemo which has definitely slowed down recovery.

My boy just got amputated. His back isn’t very good but he takes joint sups. Not amputating was a thought bc I did not know if he’d do well and I still don’t know. I think it’s a risk we take especially with old dogs. I don’t think you did a bad thing in amputating

I would have him resting and monitored. If it doesn’t get better you’d probably have to go back and have them take a look. Decide if you have the funds to cover possible acl tear. Unfortunately those do not recover on their own. The pain all of the sudden might be the anesthetics wearing off. Also call her surgeon and they’d be able to direct you if that’s normal like the random pain 2 days later

They can go over your options. I’m sorry this is a nightmare

Try to look on Reddit, there are tons of stories that may fit your own. My gut feeling that it would be have to be fixed or pain management until euthanasia (non medical advice)

There’s a user here u/bbgoatbabe whose dog also has back/leg issues. Might be someone to talk to, just to get some advice as to how the decide to continue seeing as thier dog had other issues. E