Hi, from what I've read babies have a sleep cycle of around 45 minutes (whereas an adult sleep cycle is double that) - it could just be that she's naturally coming into lighter sleep, (which I notice with my 4.5 month old) and can't stay asleep as she hasn't yet learned to settle herself back again at such a young age. You could try settling her again and see if she'll get through another cycle... which she may well need at that age.