My X had an affair and 'breakdown' in conjunction too, funnily enough, to the extent he was very nearly sectioned by his mother. My preference was dissection but neither step was taken!
There seems to be a pattern forming?
He was aggressive when found out as well.
Re. your OP, Startingover: "I'm just wondering was his reaction because he was angry over being caught or angry over being accused in the wrong?"
Both! He will be angry over being caught and being accused in the wrong because it wont be his fault (to his mind).
Don't be drawn into his confusion or agressive exchanges! It will be in part genuine, in part an attempt to throw you.
2rebecca, there is such a thing (illness) as a breakdown, it's dangerous to say otherwise! It is otherwise known as a 'crisis' and is the point at which suicide becomes a higher likely outcome. Even if he has had/is having an affair and this state is the fall out of that it doesn't actually make it any less valid.
If you are genuinely concerned about his mental health startingover, call your local community mental health team for advice, they should have an on call social worker available.
Aside from anything you will soon receive confirmation of wether he is bluffing in any sense, or not.