Are you experiencing a "headache" or "headpain"?. I didn't have headaches after my retrosigmoid, but I did have what I describe as very superficial "headpain" (i.e. it didn't feel like it was inside my head, but rather superficial, starting from the base of the neck, towards the ear). Same as you, the pain didn't start right away, but I think that it started when the sensation started to come back on the back of my head. This lasted for quite a few months. I did the ice packs as Syl mentioned (I actually found this advice from one of her old posts), before going to bed. I also had shoulder/muscle pain, for which I used warm compresses.
I took quite a bit of non-steroid antiinflammatories, like Aleve and a product called Tylenol for muscle and joint pain (or something like that). What really helped me was an orthopedic foam memory pillow. I couldn't sleep on a regular pillow for months, as I would immediately start to be in pain, but this foam memory pillow saved my life. I am much better now, and I can sleep well even using the regular pillow.
I hope your pain will go away soon.
Marianna