What Causes Stiffness In Back, Hands And Legs Post A Bout Of Flu?

I had the flu from tuesday to sunday. I felt better got up and went out to eat on Sunday with my girlfriend. I noticed while I was out my neck was tight, my fingers were starting to tighten. by 2am that evening I was on the way to the hospital both hands were numbing my joints were achey and wrist were hurting now. The hospital ran test said they saw nothing, they gave me an anti inflammatory shot which helped a little but tonight my whole body feels like its tightening back, hands, wrist, legs just achey and tight can't really bend fingers. what could this be I'm afraid I'm gonna die in my sleep