B.W.
this is normal. Babies don't wean themselves under a year old, they have nursing strikes. So if she stops nursing, its NOT weaning, so don't think that. Thats what alot of people think and they stop nursing thinking baby is weaning when its simply a nursing strike. Www.kellymom.com and search nursing strike.
It sounds normal for a 6mo old though, the siting up nad then diving back in. She's just practicing her new skills and seeing what happens when she does this or that.
As long as she is following her own growth curve, she si just fine! Babies come in all shapes and sizes, and your doctor isn't concerned, then you shouldn't be either. Nurse her when shes interested, before naptimes and upon waking, and at bedtime, and whenever else she 'asks' for it.
My youngest was a speed nurser. Even when he was a newborn he could empty a boob in less than 10mins. He nursed until he was 2, and he never did nurse more than 15mins. She's jsut efficient. :)