It was rumored that after that concert, Elvis was found sitting on his balcany at his hotel stoned out of his mind. He had let the drugs go for two months, but the pressure was just too much.
He did look wonderful at the show and he moved enough to invoke some reaction. He evidently had gotten used to not useing the muscles in his legs and arms and it caused him some immobility. People wanted to see him go down on his knee, lean back, swivel his hips and jump around and he just was not able to do that. But he still sang great and I tink was the best looking and beautiful person in the world as man others did at that time.
I believe Elvis had mental issues (bipolar perhaps, chronic depression) and also physical dealing with the liver disease his family had genetically - although drugs and drink make it worse - his MOM drank, her liver disease killed her. Elvis's colon was evidently huge from "back up waste" and with liver not working properly it caused lethargy and confusion just like somebody with beginning of dementia - he evidently did have a lot of pain - problem was he DECIDED what meds he could mix - and he did become severely addicted to the drugs and the good feelings he got from them.
Many people today are addicted to their drugs, they just don't have to travel all over the US and entertain screaming audiences - they are not junkies, just people in pain who need their meds..
Some side effects of meds certainly could be insatiable hunger causing weight gain along with fluid retention and backed up colon (HORRIBLE SWELLING from medical conditions though) not true fat...