Asking for a friend, her gelding is suddenly v lack lustre and a bit off his food (well I should say has slowed down as loves his food and wolfs it usually)! Could this possibly be linked to his jabs 1 week ago?
Suggest to your friend that she looks for swelling at the injection site - it's not super noticable immediately but will make lowering the head painful. We had a yearling ended up with a swelling about 9" circumference - initially quite hard. IF that's there, hot foments and massage can help reduce it but it MIGHT take months to go completely.