lab attacks 1 dog

Question
I have a female black lab that will be 3 on August 10th. She has been spayed already. Just recently she has started attacking our 16 year old Pit bull. We have 4 dogs in total all diffrent breeds. The only dog she attacks is the older Pit bull. Never has been aggressive towards anything or anybody. This has happened 3 times in the last couple of months and as far as anyone knows there is no cause for this. Not sure what is going on. Really dont want to have to put her down but she hurts the pitt very much and has put hles in her stomach. Last accurance was yesterday and the pitt bull is hurt badly. What could be causing this. The pitt bull can not defend herself anymore.

Answer
I am assuming all these dogs lived together before with no trouble?

Has ANYTHING changed in the home?  From as simple as changing dog food to someone moving in or out of the house?  ANYTHING???

If nothing is different in the house, I would take the dog to the vet for a full work-up.  Sudden changes in behavior are usually medical.