What is the reason for headaches after PRP 4 months ago?

This is not typical. If it persists, I recommend following up with your Doctor for a formal evaluation. Keep in mind, prp is effective, but I have a very specific protocol, every 3-4 weeks for 6 months with prp therapy. I also add in progesterone to speed the healing process.  After that the injections are tailored down. You should also be on viviscal and other growth promoting ingredients. See an expert for a formal evaluation. Best, Dr. Emer

This is extremely unusual and I have not experienced this in my patients. I have about 5 years of experience using PRP. It is possible that a nerve was injured during the injections. It might be possible there is a fluid collection or infection. I would seek another evaluation. Sometimes if I inject ACell, there is prolonged nodularity as this is gradually absorbed.

Headache is common after PRP treatment but it usually subsides in a few hours. Your headaches are likely not related to PRP treatments and may need further work up by a neurologist. Best wishes. 
Razieh Soltani, MD, Board certified dermatologist 

You will need an in person evaluation but this could be a hematoma causing pressure on a nerve or it could be a swollen nerve that was hit in the procedure. In my office, I would use a high resolution ultrasound to determine if it is either of those or something else. There are good options for treatment depending on the diagnosis but you should not wait any longer to have this evaluated. 

I have not experienced this in my practice nor have I heard this happening. I highly doubt that injections of PRP would cause headaches lasting this long. My advise is that you see your personal physician for a full evaluation.