I lost 2 of my piggies this year, one in September and another in October. Susie bless her, died due to not being strong enough to recover from her surgery and Heidi my first born, died of what we think was cancer.
Now, when Heidi took ill she declined rapidly over a week, we were at vets nearly everyday. The last two days she wouldn't eat anything at all and lost weight by the hour on her final day. On her last day we took her to the vet again and they said the only thing they could do was to open her up and find out what was going on.
Now I knew my baby was too weak and wouldn't have survived surgery so I decided to take her home under my vets advice and try to pick her up and get her eating again before going ahead with any surgery.
Unfortunately, she died in my arms the next morning. Before she passed, she was unable to walk and her entire body went floppy as you are describing. This also happened to my Susie aswell. I can't and don't want to tell you what you should do as it is your decision but I look back now and wish I had ended her suffering and considered euthanasia. At the time I just could not do it, just thinking about it broke my heart in two but it also shattered my heart to watch her deteriorate.
I am so sorry this is happening, I really am praying
for you and your little one.