Sudden aggression in puppy

Question
Hi Carol, I have an 8 month old Parson Russell Terrier who has been the sweetest thing since we got her at 9 weeks. We have had her in obedience, leash skills and puppy playgroup since about 12 weeks, and she has always been a doll, the behaviorist that monitors the playgroup even talked about the possibility of her being a therapy dog (which I wasn't necessarily interested in, but it illustrates how totally sweet she is, especially for this breed). Well, we went to playgroup yesterday, just like normal, and she was super-aggressive with two other dogs, both dogs she had played with before, and we had to take her home. I thought it was the strangest thing, and the behaviorist was completely surprised too.

However several things happened recently, first that she was ill over the weekend (she was cleared by the vet and the behaviorist to return to playgroup), second that the week before last we began using a correction collar in leash skills class, though she didn't seem to mind it and responded really well in class, and third that she caught our neighbor's chicken in our back yard on Sunday (I got to her just as she caught it, so she didn't do any damage, thankfully, but it was a totally new experience for her).

I am praying that she just still wasn't feeling well after her illness this weekend, but I am terrified that this is the new normal for her. If it was that she was still feeling bad, is there a chance she will just go back to her old self when she is better? Or could the correction collar or the chicken incident have changed something about her? Is she being a teenager and developing a surly personality? I am not sure what to do. The behaviorist said we could try again cautiously next week, but I am totally freaked out.

Thanks so much,
Lisa

Answer
Lisa,

Pups are still growing and forming their personalities for up to a year and you have a great deal of time and effort put into this little girl.  I think she probably just had an abundance of energy on this particular day from being cooped up in the house sick.  Try to take her to the park again, but before you go, make sure you walk her or run her to burn off some of that excitement/energy ahead of time. This is a recommended suggestion to anyone who will be visiting a dog park.  I doubt very much the chicken had anything to do with it - and stick to Martingale collars. ANY correction, chain or spiked collars are bad news.
If she acts up again, immediately remove her from the park and repeat this over and over, she will eventually learn that behavior reaps no rewards.  
Aggressive behavior just can't be tolerated - especially at the dog park.  
Best of luck, and good job with her!!!

Sorry I didn't answer sooner, my dog had medical issues this week, been tough and I
haven't checked my email in 2 days.  

Take care