Back when I lived in a more rural situation, our cat was an in+out-door cat too. One morning before work I noticed his nose was runny, so I made sure he was inside for the day. Came home and he was sitting in the closet, hadn't eaten, grumpy & lethargic, poor guy!
Have no clue what caused it...probably some sort of virus, who knows. Antibiotics fixed him right up though.
Glad your fuzzy guy is feeling better!