L_of_Shame said: Sweat makes me think they were doing something... That causes sweat to perspire.
She's possibly sick and lying in bed all day. The process of healing can be really taxing on the body depending on how bad the ailment is. The existence of this trope - I've seen it a few times - is probably related to Japanese cleanliness habits, hence why she doesn't just sit-down-shower (is there a word for that traditional pre-bath prep?) or take a bath when that sick.