I had the same problem with my pin, Connor.  He developed doggy-style hayfever the first spring I had him and the vet told me to give him benadryl.  Worked like a charm.

 

Two years later he came down with hives.  I rushed him into the vet and after looking him over really well she guessed he'd been bitten by a spider or something and gave him a benadryl shot.  She told me to keep giving him benadryl for the next 24 hours or so.  The next day the hives came back.  It took two more visits before we figured out the truth.  Connor's allergic to the red dye [i]in the benadryl tablets[/i]!!!  The very medicine I was giving him to make him feel better was actually making him sick :(.

 

So I switched from the cherry-flavored pills to the grape flavored ones (different red dye heh).  No more hives.