Stress can definitely cause or aggravate allergic reactions and yes, that can result in itching.
After an OIS at work, I nearly lost my mind from the sudden exacerbation of allergies... on a level I'd never seen before. It started several days after the shooting. The worst of it... the symptom that made me talk about shooting myself in the ear.... was those God Awful ITCHING EARS. That went on for months and I was so nuts trying to get at that itch. Just miserable. Fortunately at about that time the "next generation" non-drowsy anti-histamines came out and I took those until I got help through NAET.
I didn't realize at the time but the stress/allergy connection is not new or unknown. When you think about it, it makes perfect sense. Allergies are the manifestation of the body's fear that it is being attacked. I mean that literally, not in an airy-fairy-Louise-Hay way. Your body thinks it's being attacked, so it goes into defense mode. When you're "stressed," you think you're under attack and your body goes into a defensive mode also. Some of the manifestations of that are different, others will naturally overlap. Stress has a lot of other really bad effects on the body as well.
As far as "why now?" it may simply be that it takes a little time for stress to take that toll, or it may be that the combo of stress and pollen is simply too much at its peak.
Give Jack some extra loving and exercise/play. Talk to him and tell him how much you value him. Intent matters. Massage sessions would also be good. And stop and think about what you said... "with my luck, he'll live another ten years!" If I were talking about a dog I loved, I wouldn't say that. I might say "If I'm very lucky, he'll live ten years." I realize it seems irrelevant and Jack doesn't read the message board, but check what you're really thinking and focusing on because it IS in your energy and he knows it. And I know that no matter what you say now, the day Jack dies you're going to be heartbroken, so I'm not saying you don't love him. It's just that sometimes we get into thought patterns or speech patterns that express things that aren't really true and aren't helpful.
Signed
"the woo-woo squad"