Why is Hazel taken to the hospital in Chapter 7? What does she learn about her condition when she awakens in ICU?

Hazel screams because her head is killing her in the middle of the night and her parents take Hazel to the hospital. Hazel wakes up in the ICU and when her parents come in, they say her headache is from deoxygenation form the fluid that had filled and now was drained from her lungs.