Yeah in my experience it is blocked piti. Probably you are habitually contracting somewhere like the neck or face muscles which is causing the headache. In my experience it can also be caused by an excess of thinking or self-identification. Figuring out how to relax those contractions takes a while because you have been doing it for so long you are not aware of it. Becoming more aware of the lower body helps to drain some of the energy downwards. I have found Bhikkhu Buddhadasa's book Mindfulness of Breathing very helpful in elucidating the links between the breath, the body and the piti and how you get them working together. I am still actively working on it, so I'm probably biased, but you can check out more details on my log.

Remember, anapanasati sutta advises stuff like relaxing (tranquilizing the bodily formation) on the inbreath and outbreath. In my experience if you stay on that track of letting go of body tension, it's possible to relax to an extent I did not think possible (not just mentally at ease, but actual muscles extremely slack). This seems like a very good thing.