“But when the sick feeling goes away and I open my eyes, the red sweater is still sitting there like a big red mountain.” What does this mean

Need context really to what the red sweater means to the person but it’s basically saying even when they stop feeling anxious/ ill the “red sweater” (whatever this means on context) still sits there like a big problem. You can infer the red sweater a big problem through the word “mountain” and is something they need to overcome. Red had connotations of “love” or “fire” maybe. Depends on context. Hope that helps :)

Evoke strong feelings, to create an impression, or for comic  effect. it is not meant to be taken literally.  hyperbole  is quite common in poetry and everyday speech to convey emphasis.