The headache stopped by DR in Cold Days only started in the room of "Memory... Reflection." Kinda supports the idea that the headaches' purpose is to keep him from certain trains of thought.
The immediate thought Harry had right before the "parasite" headache in
Cold Days was in recollection of how Ebenezer reacted to him laying down a Sanctum Invocation on the island. He thought "stunned"...then "horrified." Then *BANG*...one of the double ice pick headaches instantaneously.
Technically, I think Harry has the potential for
at least four things playing with his head.
1. Mab...when she gets a whim
2. Lash, if she's still in there
3. Gained knowledge from his Mother (Blood Rites) that might or might not have activation protocols upon recognition.
4. Gained knowledge from his Mother (Changes) where he receives another gem containing knowledge of the ways.
5. Perhaps an unknown party known as the parasite (my thoughts lead to Arianna's death curse).