Thanks for posting your query.
From which site Pacemaker probe was introduced? Whether femoral or jugular/ radial.
If jugular route was used then there may be chances of neck involvement and in such cases, patient may have obstructed vein, severe headache
with swelling at face.
If neck conditions are normal, then there may be high chances of muscle stiffness
as already explained by your treating physician.
You should again consult with physician and go for color Doppler study of neck to see patency of neck veins.
Your father should also take muscle relaxants if all prescribed investigations are normal.
Get well soon.
Dr. Mayank Bhargava