Everytime after doing yoga in lie-down positions, when I sit up straight, I have a hammering headache. I don't understand the reason. But if I continue to lie down for about 20 mins I feel perfectly fine. Why does this happen? Any docs in the group?
You issues may be cervicogenic. That means neck related. It will go in few months of regular practice unless you have really vulnerable neck issue. In later case, you need to get assessed by expert in yoga therapy and get it fixed first.