Being stressed is not the same as being tired or being overworked or being anxious. It is close to being pressured or hassled except that stress need not be produced by people's demands. Bereavement can cause stress. Moving house can cause stress. Difficulty with staff can cause stress. Dealing with any difficult situation can cause stress.
In theory you can reduce stress by refusing to deal with stressful situations. But when you are hassled and pressured you cannot escape.

Like many of the mood codes, 10/19 is a plea for understanding: 'I know I am being difficult but this is because I have a lot of stress at this moment. Bear with me.'

Having moods is not so much a sign of weakness as a sign of being alive. Signalling your mood prevents misinterpretation of your behaviour.