Is normal after having injection bb won't have good appertize as normal. For some bb, this will last for 3 days MAXIMUM.
(My bb have injection last Friday (2M) Nothing happen, everything same as normal)
Regarding the red dot on the neck and ear, have nothing to do with the injection. It is because the milk liking to those part or may be he is too hot. You better use warm water to clean after he drink milk each time. This will get better after 100days automatically.