
As others have said, 6 weeks is awfully young to be taken away from her mum.
I picked both of my current girls at 6 weeks(selected which one I would like to have at 8 weeks)and they were definitely too babies to leave their mum at that point!
Not that the breeder would have let them to be taken at that age anyway.
I have Leos so similar sized breed and my first Leo nearly 20 years ago came home at 7 weeks and weighted 9.9lbs so it is possible your girl is 7 weeks old if Newfie weights are anything similar to Leo weights(I would think they might be little heavier at this age tho, compared to a Leo)
My current two Leos have weighted 22lbs and 17.6lbs at 8 weeks so a lot more than my first one!
Even though she weighted less than half what my 22lbs girl weighted she was not skinny(e.g. not fed enough, wormy etc), just small in every way.
She was not small compared to the rest of the litter, they were all similar size so she was not "runt" or anything.
What did the breeder feed and advised you to feed your puppy?
Looking back my first one was tiny and too young even at 7 weeks, that extra week makes a massive difference but we did not know any better at the time.
Have you got friends/family with friendly dogs?
As one thing she will have missed out on leaving her mum so young is lessons in doggy language and behavior.
I would say it is extremely important you get her mixing(after she has had her jabs) with friendly adults who will teach her doggy manners.
Maybe you could try and see if there are Newfie people meeting up somewhere near you?
If you google newoundland weight charts quite few come up giving you average weights for different ages.
Looking at these your puppy does seem very light...