It might be teething. Or it could be an upset tummy - has she tried any new foods today? Or she could be hot? Cold? Or she could just be being a little monster, especially if she's happy and content in certain positions. My little one (8 months) has been doing this to me a couple of times a week since birth and all I do is either get reaaallly comfortable in the rocking chair or in front of the telly and (if it's a weekend) take shifts with DH.

Hopefully someone else might have a suggestion, but I hope she gives in soon and you get some sleep!