Does my girlfriend have a personality disorder?

[CompletePimp]

We've been together for 6 months now (I'm 22 she's 25). But she's been having this strange behavioral pattern the entire time. Where she would get really close, open, and mushy. Then something would click in her and she would push me away, be really distant and talk to me practically like I was a stranger. Then after a week or so she's all mushy calling me sweet names and being open again. Then a little while after she shuts off again and talks to me like I'm not even her boyfriend, completely formal and really distant.
This isn't PMS I'm pretty sure. Does she have a split personality disorder? I broke up with her over this a few months back and when we talked it out she really compensated for it and became really sweet but then again she shuts off on me. She says it's not me (typical) it's just that she feels she is overwhelmed sometimes if things get too mushy and isn't completely ready for a really serious relationship. Yet here we are, in a committed relationship that SHE asked me to be in, 6 months in. What am I supposed to think of this bizzare behavior? We only spend weekends together due to schedules and slight distance. So how can one feel overwhelmed? Should I just back off of her and let her come to me? I'm kind of sick of chasing her around and always asking what's wrong when I don't do anything wrong. And it seems like this is a pattern, where if I don't do anything about it really she just snaps back into normal.

If you need more details please ask. Thanks in advance!

edit: Also, it seems like this flip happens when something stressful or anxiety-inducing happens in her life - work, friend issues, house payment issues come up etc. Like she thought this week that she might have been pregnant because she missed her period (got off the pill, it's a side effect. thats why, I kept telling her) so she shut off this week on me. Turns out shes NOT pregnant of course.

[Vain]

From what you said it seems like to me like this is just her way of dealing with stress. My ex was similar. When she felt overwhelmed in the slightest she would become distant and her words would be few. I know it doesn't seem like your relationship is overwhelming but it is relative right? To her it might seem overwhelming, even though you see each other only on the weekend.