I think you should go be a reader. I had my surgery on Tuesday and was singing at a fundraiser on friday. ( I should add I'm also older than most here!) I was tired but took it easy which is what you should do. Sit and visit at the reception. Have a drink with you to sip on. Others have suggested just saying you've had an upset stomach if anyone asks and I think that's a good idea. Maybe I'm odd, but I didn't have shoulder pain, didn't need pain meds after I got home and other than being tired more easily, my recovery wasn't hard. Good luck.