John had this problem but a visit to the dentist many months later solved the mystery. He must have been clenching his jaw and grinding his teeth due to the stress and this was apparent in the wearing down of his back teeth which was apparently quite noticeable to the dentist. She said this teeth clenching was the most likely cause of the headaches. His chiropractor, on the other hand, thought the headaches were most likely caused by the tension in his neck & shoulders ... again, entirely stress related.
If you take all your tablets to the pharmacist, they will tell you whether any would cause your problem.
I hope you get it sorted out soon.