From Louisa's sudden fatigue after reading the note, Harriet must have dropped a hint that she wouldn't be coming over on Sunday.Compare other situations where Louisa's emotional reactions have similar somatic reverberations.
See, for example, O10:
"I feel guite lonely now Harriet is gone. I shall retire early tonight; I do not feel well."And N21:
"Nothing of any consequence has happened today. I am [a] little disappointed at not receiving a note from my friend this week. - I feel rather tired after my day's work and shall retire early.")