This happened to Cinnamon after her first set of shots. We got her home and she was fine she was playing witj my nephew then she suddenly went stiff fell over and couldnt move. Ill never forget the look on her face--her eyes just went bug eyed she looked terrified. We rushed back to the vet and found out She too took an allergic reaction, not as severe as your pup mind you, but a reaction nonetheless. She now has to be pretreated with benadryl before she gets any shots. Half a tablet the night before half the morning of half the night of then half the next morning.
Glad to hear Riley is ok though