<s>[INST] I was lying on my back and almost immediately after I turned onto my left side, the right side of my brain began to pulse. It was only temporary, ending almost as soon as I sat up. I thought it was a lack of oxygen so I began to take in deep breaths. Its gone now, but Im still a little concerned. [/INST] Hi, sudden feeling of pulse in brain is throbbing pain felt in scalp, due to irritation of trigeminal vascular system due to any cause, since it has disappeared, appears to be episodic or paroxysmal, which need to be evaluated. Brain as such is painless structure, its covering if effected by neighboring structures only produce such feelings. Kindly show to nearby neurologist for clinical examination and evaluation. Regards Chat Doctor.  </s>