can anyone share some experience with this please? i have been feeling like there is a tiny cold stream is flowing in the top of my head (which is odd and pleasant) accompanied sometimes by intense left side head and neck pain. it comes and goes. i took the mudra out of my practice and have started noticing a much lighter version of this pain after nauli before starting with pranayama. any advice would be much appreciated.
Yoni Mudra Kumbhaka is a powerful practice which will bring a lot of prana into the higher centres. It will especially work to activate the ajna chakra, but may also, through osmosis, have an effect on the crown chakra. The experience of the tiny cold stream in the top of your head is related to the activation of the crown chakra. More specifically it is a flow of prana through the Brahma nadi, which is a nadi that runs through the centre of the crown chakra.
If you are experiencing pain in your head and neck then that is a sign that you are moving ahead too fast, so you were right to self-pace by leaving out YMK for now. If you are getting pain from nauli then that could also be an indication to leave it out as well. The art with self-pacing is to find a level of practice that we can do without experiencing negative symptoms or pain.
Remember also to stay grounded, through engaging in grounding practices such as walking, every day.