Cat acting strange after a night outside?

I have an 8 month old neutered cat who spent the night outside, as usual. When I usually let him in, he's energetic, vocal, and plays with my feet. Today, however, he seemed solemn and withdrawn and even scared and isn't as interested in playing as usual. His collar, which was on pretty snug, is missing. There doesn't seem to be anything physically wrong with him either. No limping and I can pet or touch him where ever. He is cleaning himself a lot more though. What would cause him to act this way? Maybe a fight with another cat?