Character A is the type who hides&denies being sick if they are around people, unless they feel comfortable enough being around said person/people. So after a day of being around a lot of people A is left alone with B and they just collapse on a chair and give in to the exhaustion and the coughs they’ve been fighting back… The change is sudden and really obvious and B is surprised. They hadn’t realised just how sick A was and they hadn’t expected A to act so differently once they are alone. B has no idea A considers them “safe” and/or “comfortable” and that they think it'sall right to be vulnerable in front of B.