Re: sharp pain in left side? help please!
I don't know if the standing up has anything to do with it other than maybe putting different strain on wherever the pain is coming from so that you don't feel it when resting, but one thing that causes pain on the left side is Diverticulitis. I think the pain caused by that might be lower than you described, though.
I think it's the best idea to see a doctor, and they can order tests and scans to be able to see what's going on in there.
Keep us posted!