Both. It stems from the mind, yes. But the brain is not an isolated “thinker” in your body, it’s the central hub of a very complex neural system that spreads over and covers every inch of your body. So when the whole of your neural system is rewiring itself into its natural, nascent state, there will be a lot of bodily sensations, and it will feel very physical, yes, even extremely so. It is not psychological in a sense that it’s not coming from your psychology, it’s coming from dismantling, falling away of your psychology. But it can be described as neurological, that’s why all Kundalini rising ceases eventually in enlightenment. When your mind and your body are whole, when they are finally one, there is no more sensations out of nowhere, just what you see, touch, feel. But with a totally different quality of wonderful sensitivity.