Terri- I was always told that when an infant is in a situation they don't know how to handle, they go to sleep. It's their stress release mechanism. My oldest used to scream for 10 seconds after a shot and then go right to sleep.
Kids are weird. I'm just waiting for GS to get up- they stayed over last night (furnace issues) and I want to make him a good breakfast. (He's 19 mos. old)