i slipped and fell in costco on food on the floor, later in the evening had headache. next morning severe headache.

Just because a person is injured on a business's property does not mean the business is automatically liable.

Each person has a duty to protect oneself. That is, did you watch where you were going?

A business must have notice of the danger. That is, was the food on the floor long enough that the business had an opportunity to notice the fallen food to clean it up?

Many businesses have insurance that will provide some medical expense coverage to injured customers regardless of fault. You can check if the business has such insurance.

You can review your specific facts with some local attorneys.