Maybe a developmental shift? Perhaps she has a little bit of something (like a cold?) that's making her body want to rest in order to get rid of it? Maybe she's having a growth spurt or learning HEAPS at the moment and its making her tired... Has she been eating more too? Maybe its a freak couple of days and she'll be back to 'normal' again next week? Just some ideas.
Natty's sleeping patterns changed pretty dramatically at around 7-8 months, but that was when we night-weaned, so I put it down to that... Which isn't much help to you, sorry!
Is she otherwise well?