rubys behaviour!

So basically when i walk ruby with my other dogs shes fine with other dogs, however shes not always okay with other dogs when i walk her on her own, she went for a dog the other day a large boxer, its the first time ive ever seen her show aggression to another dog as shes normally very submissive and playful, she was on her own, why did she go for the dog? Should i be worried? Shes never shown aggression before, she is normally walked with my other 3 dogs and ive never had a problem, i decided to take her for a second walk as the others are older than her, i originally thought she may be a full staffy but shes definitely crossed woth a jackrussell, she looks like a slimmer staff, she doesnt bark or growl but she did when she went for that boxer, help and advice would be appreciated guys :-)

How old is Ruby?
Perhaps she was scared being on her own. What was the situation like with Ruby and the large boxer? e.g. were they both off lead? etc

Shes 1 year nearly 1 year 1 month, ruby was off lead the boxer was on the lead, everything was fine at first then all i seen was ruby with her hackles up trying to bite its neck and face, she didnt make contact as i shouted her name and she stopped, i was talking to the owner

So the good news here is that she stopped and responded to you

See how she gets on over the next few days, it could just have been a one off but if not reinforce the recall so that you can avoid any future conflict.

Yeah shes pretty good with the recall, i will definitely keep it up, thank you for yours advice