Several options mate.
Maybe she initially believed that she could handle a open relationship (aka sharing you with others) without being too emotionally involved and suffer from that situation. And obviously it didn't work, and she got too attached, even without her consciously knowing it, for giving you to other girls. That's the best scenario.
On the other hand, she might have wanted an open relationship "for herself" (understand that she never really believed that you would take advantage of that) and she's upset because she was wrong, and she doesn't manage to tie you up to her alone. The fact that she still wants an open relationship while kind of preventing you to go somewhere else might go for that case. But still it's the "worst possibility"
I'd advice you to stop that open relationship. And be prepare to end all form of relationship if she gives more credit to her liberty than to yours.