Sorry to revive an old thread, but I've experienced something similar and in my search for answers I "came" (heh) across this post.
I've been on the recieving end of various dog cocks for about 23 years now. I have never had an allergic reaction to anything my entire life including; food, pollen, animals, and medication.
The last time i let my GSD mount me was sometime last year because about 10 minutes later I had itchy red raised hives covering my entire body. Roughly 10 minutes after that I noticed I was having difficulty swallowing and my breathing was labored.
I took a Benadryl at that point and probably should've gone to the hospital... but i was too afraid because I thought id have to tell them what caused it. My throat started to un-swell about an hour later, but the hives lasted for around four hours I think.
Nothing like that had ever happened to me before with any other dog (multiple breeds) or in my five years with this one.