I'm going to side with Tan on this one. From what you are saying I think it is developmental, and perhaps a little bit of seperation anxiety (it's supposed to peak around this time apparently). Unfortunately I don't have any magic solutions for you, but perhaps you could leave a piece of clothing in her cot, one that you have worn (so it smells like you) and might help with some anxiety on a basic level.
Did Beth go through similar sleep disturbances at other developmental milestones (crawling, rolling etc)?
I'm not sure how you feel about leaving her to cry it out a little, so I'm not going to go there. But I know that when
DD was waking several times a night at about 4/5 months (and when I say several times a night I mean over 8!!! I never got more than 2 hours of unbroken sleep) it got to the point when I couldn't handle it any longer and let her cry. You will know what is right for you and your
DD...
Wishing you lots of luck. I know how frustrating it can be.