Perhaps something happened when you were asleep? does he sleep with you? maybe you did something to him inadvertently while you slept? my tiny guy always tries to tangle himself in my legs under the blanket at night but he has figured out over time that I move a lot when sleeping so he has figured out that it's in his best interest to sleep near my head, now most mornings I wake up with his head in my neck LOL his legs there and his body curled around my head, it's funny!
Anyhow, also what MiniGrace said could be very true as well, my male hates it when my husband wears a baseball cap.. ha ha he barks his little head off at him until he can sniff him & a few weeks ago I was wearing a hood from my jacket and he even barked at me.. so sometimes they do get spooked from the silliest things!
Gotta love them