system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:What is going on in your brain when your have a headache/migraine?

Why does you're head feel like its pulsating?
Why do you become sensitive to lights, noises, even smells?
gpt
While the brain itself doesn't have pain receptors, many of the tissues surrounding it do such as the skull, blood vessels, eyes, sinus cavity, muscles, etc. When you have a headache, it is generally some sort of pressure on those areas that do have pain receptors. 

You feel the pulsating feeling because of your heart pumping blood to the area with pressure. With each heartbeat, the blood flowing to the area adds to the pressure, periodically causing you to feel the headache.

The sensitivity to different stimuli can occur because each of those senses activates a region in the brain. When a brain region is active, it requires more blood flow (this can be imaged using functional brain scans). If the headache is caused by pressure near those sensory areas on the brain, then additional stimulus to those senses will increase blood flow and thus, increase the pressure to those areas.