If it's come to this, your girlfriend's already suspicious that you're suspicious. And guess what, Sherlock? You're probably on the right track.
We personally think that social media passwords are like pin codes: They should never be shared. Ever. Some things need to be yours and yours alone. It's information that, if a friend were to become an enemy, could be dangerous. And make no mistake about it, an ex can easily become an enemy. However, when a relationship's lovey-dovey stage is at it's absolute pinnacle, often times smart decision-making goes out the window. Passwords get shared.
Now, if you suspect that your lady may be up to no good, and then you find out that she's changed all her passwords, well, let's just say that nine out of 10, where there's smoke, there's fire. Think about it. Why, short of identity theft, would you suddenly change all of your passwords? What other justification could there be?
You're getting played, player.