Behaviour change in Choc Lab Female

Question
Our 2 1/2 year old Choc Lab Female, has started to become aggresive with other dogs.  Never the same,well there have been 3 dogs so far, and never with any warning.  She is walked 3 days a week during the day with the dog walkers Staffie and we walke her every morning and evening everyday.  She has been spade, nearly a year ago now. Last week we were informed that she had over 2 consecutive days 'gone for' random but local dogs on the downs where she is walked everyday.  She has NEVER done this whilst we have walked her and we have never witnessed these events, she plays very well with the staffie she is walked with, they can get a bit lively but they never growl, bark or snarl when playing with the each other.  I assumed that she was becoming jealous of other dogs who may go near the staffie as they are very close, but today our walker said whilst she walked her alone, without the staffie with her, she attacked another dog.  When we (alone or as a family) walk her, she is off the lead, when she approaches or is approached by another dog she has always been very sociable.  Could she be trying to protect our walker, could she be jealous, could this be something they go through at the time when she would have normally had a season???  I am very worried and as yet the other owners have been very understanding as so far she has only scared their dogs into submission.  I have asked that our walker lead walks her for now and have made an appointment for our vet to check her over.  This has NEVER happened before, she is very gentle with us our son and all other humans, she is a very loving dog, lazy but loving, yes she can have selective hearing when we call her back but she has never seriously disobeyed us so that we would consider classes or anything.  Should I even stop her walking with the Staffie??? Help!!

Answer
Hello, Is it possible Her dog goes after strange dogs sometimes and your dog is just catching on now? Also it is indeed possible she behaives when mommy or daddy is around and disobeys when your not there. Yes I highly reccomend she is walked on lead from now on or at lest when your not walking her yourself as some day an owner wont be understanding and may order your dog put to sleep via a court order...sounds awful but its not totaly impossible even for great loving family dogs like labs. I note that when I am walking my lab by herself on leash...always on leash she sees another dog or hears one will flatten her ears whine and pull ****Away...if I have her and my other dogs also all on lead) she will get agressive if she spots another dog or hears one and goes for it....she is in my mind protecting her smaller siblings(not really related). She is 3 years old and also Spayed since she was around 11/12 months old. I am really very sorry I cant be of more help but this is really nothing I've ever heard of before.