Has happened to me a few times. Something about the mob and lag from what I've gathered from what others have said, in this case, the Armax not realizing they are dead and continuing to attack.
You had the chat error in your chat and thats why you got damage. The damage came from the Armax, it happend for me too on Argonauts, I killed them and after they were dead I started to get damage after damage.
I am pretty sure it was delayed damage lag. We were getting it very badly over the weekend during WoF. I revived and while I was healing I died again from a hit and mob was no where around me.
I take my pets in water all the time hunting fishes and they have never attacked me.