Pneumonia perhaps, but I would think they would have kept him in the hospital Friday if that were the case... unless he refused to stay because of his Saturday appearance. Some people work too hard and refuse to slow down when needed. Who knows. Guess we'll hear more in a few days.
I suspect if Prince was adamant in not being treated, nobody was going to tell him “no.”
My guess is heart attack. Prolonged flu and performing gigs is a huge strain on the vital systems.