The person who notices you have gone quiet should leave room for more than one explanation.

Silence is one of the most misread signals there is. It can mean hurt, or tiredness, or concentration, or a hard week that has nothing to do with the person noticing. The noticing is kind. The trouble starts when a single explanation gets chosen quickly and the whole conversation is built on top of it, so that people end up defending a mood they were not having.

If someone has gone quiet, say what you noticed and stop there. Let your question be open enough that the true answer fits, including the answer that nothing is wrong. Believe the reply you get.