It is not uncommon for a horse to resist putting their head down after an injection. This can be due to a few different factors:

1. Pain: If the injection was painful or uncomfortable for the horse, they may resist putting their head down in an attempt to avoid the pain. This is particularly true if the injection was given in the neck or shoulder area, where the horse may experience muscle soreness or stiffness.
2. Fear or Anxiety: Horses can become fearful or anxious after receiving an injection, especially if they have had a negative experience with injections in the past. This can cause them to resist putting their head down or moving in a way that they perceive as risky.
3. Discomfort: In some cases, the medication given in the injection can cause discomfort or nausea for the horse. This can cause them to resist putting their head down or engaging in certain movements.
4. Behavioral Issue: Finally, some horses may have learned to resist putting their head down after an injection due to a behavioral issue. They may have learned that putting their head down leads to pain or discomfort, or that it is a way to avoid being ridden or worked.

If your horse is consistently resisting putting their head down after an injection, it is important to have a veterinarian examine them to rule out any medical issues. If no underlying issues are found, it may be helpful to work with a trainer or behaviorist to address any potential behavioral issues and help your horse feel more comfortable and confident.