Ok I've come across a problem.
Friend of mine that I help train, complains of sudden sharp pains in his head when he hits a heavy bag and training mitts.

This happened a few days back. Since then I told him maybe to watch his breathing because he may be unintentionally holding back air while he punches.
He told me today he breathes properly but still gets sharps pains running through his head when he hits both the heavy bag and focus mitts.

This one has got me in a twist? I've never come across this kind of problem.
Little info on him.

He is super middleweight, 6'3", 22 years old. Pretty strong and cut.
Never boxed against an opponent as of yet.
He once had a head injury which was caused by a car accident a few years back, but since then has had many head/brain scans, all clearing him of any problems.

I...am...puzzled! Lol.

Anyone can help?