Here's my perspective, which actually has more in common with her side.
whatever you do, DON'T ISSUE AN ULTIMATUM!!!! I can't stress that enough. she could be after one of two things. Either she's having second thoughts about the relationship, and some time apart/space could be the best thing, or like most pf the people above said, she's discovered freedom and will over-do it for a little while, but will eventually find a happy middle ground.
And seriously, the worst thing you could possibly do would be to issue an ultimatum or start making demands.
Hope that helps.