Aggression to puppy after both were spayed

Question
We have a wonderful 5 yr old black Lab (Sophia) who has always been very mellow -- doesn't know the word "bite". We recently adopted a 3 month old puppy (Punkin). We've been keeping both in our bedroom at nite so the pup won't be looking for her older sister. Recently, for NO REASON, while coming back from a late nite feeding/bathroom run, Sophia attacked the puppy. We were horrified. She's never, ever been the least bit aggressive and wasn't when the pup first came into our home. They play great most of the time. but every once in a while, Sophia will go after Punkin. We don't know why.  Will Sophia stop this when Punkin gets older, or will it get worse?

Answer
It might have been that she may have been tired and was telling the puppy that she didn't feel like playing right then and there-or she may have been telling the puppy off if she was trying to be top dog

Since they are both young they should grow bonded to each other-I think it'll work itself out

I would be mored worried if both dogs were even just a year older then they are,but dogs that are raised together as pups(under a year)usually grow to be best friends