Well, you're certainly not about to drop dead of all things. Remember that the brain itself does not have any nerve endings and therefore no sensation is felt in the brain tissues. Whenever you feel spasms, a shaking sensation or even pain, realize that it involves the musculature and vascular processes outside the dura of the brain.

While persons who experience migraine headaches for instance do experience pain, it is referred pain and occurs due events occurring within brain tissue and associated vasculature. So you can relax from the standpoint that your sensations represent something wrong with the brain in the way of cancer etc. It's not the case.

Your symptoms are most likely associated with muscular spasms or muscle fatigue. Sometimes people who remain in a fixed position for relatively long periods, at the computer for instances, can develop fatigue and muscular tension that is not readily apparent but becomes chronic after a certain time.

I would not consider your symptoms to represent any sort of underlying pathology.

Best regards and Good Health