This is an absolute sign of social media shadiness. Why would all posts on your girlfriend's Facebook page suddenly require her approval? It's simple: Unless she's super conservative and doesn't want any family members or co-workers seeing how much of a party girl she is, she's probably scared that another guy, maybe even one she's already hooking up with, will post on her wall.
Think about it: If some dude writes an inside joke you don't get on her page, would you flip out? Probably. Thus, she's forced to approve all comments so that she can bat away anything that may lead to her getting caught in the act.
Bottom line: If your lady's Facebook wall suddenly turns private, it means that she's either already cheated or is, at the very least, actively flirting.