Why do i sometimee get a headache from having my hair pulled into a bun or pony tail?

Nerves: Your scalp is very innervated especially your hair follicles. Those nerves are interspersed with the nerves that can cause headaches when irritated. So, when you are trying to arrange your hair, pulling them causes a trigger in your scalp nerves that then goes on to other nerves that in their turn trigger a headache.

Muscle tension: I would imagine the headache is caused from the contraction of scalp muscles as you tighten the hair into a knot or tie. The corresponding muscle contraction causes tension, which leads to the headache.