Sciatica pain comes to mind.
It is probably from the lying. Doesn't sound like anything serious.
You can take something over the counter if it gets worse, BUT consult your physician FIRST.
I have a ton of back issues and right where your speaking of is the main part that gives me extreme pain. I cannot even sit in my chair at work anymore for longer than 20 mins at a time because I get that same pain and it starts to shoot down my right leg from my sciatic nerve. Its probably because you have been sitting for so long and not moving so I would not worry about it. If you had any back issues before that its just making it worse. Being all stiff in that bed can also make you kink your sciatic nerve too giving you extreme pain but again this will all work its self out once your up and moving around again. Good Luck.
I had a lot of wires and tubes in my arms and stuff and I didn't want to mess anything up so I only moved when a nurse said it was fine. About every 2 hours or so.