Poor little Jens.
Our little cocker spaniel, Cookie, had reactions to her vaccinations. The first time it happened her cheeks got so swollen that her snout was easily three times bigger than it normally was (she kept trying to lick her lips but her tongue couldn't touch her lips).
We didn't know/realize what it was, so we took her to the emergency vet. They gave her an antihistamine shot and she was fine. It scared the bejeebers out of us, tho!!! From then on, the vet would give her an antihistamine shot before her vaccinations and we had no other trouble. Once they knew she had a reaction, they just prepared for it and we had no other trouble the rest of her life (she lived to be 16 and this happened right after we adopted her at age 3).

I know every dog is different, but hopefully it won't be a big deal for Jens, either