ebb and flow

I always worry that bad things are going to be permanent. Like, when I get a migraine, I have a visual disturbance and then that passes and the headache and slightly nauseous feeling comes on, and I always think, what if the visual disturbance doesn't go away, but it always has, and probably always will. That's the reason for the headache. Some kind of reaction causes the contraction of blood vessels, which is the reason for the visual disturbance. Then, your brain (or body) releases a chemical to dilate the blood vessels, and it's that chemical which causes the headache.