what's happening to his sleep?!?

He's 8 months and been a fabulous sleeper since day one. We've always put him in his crib and he just falls asleep within seconds. Very easy going baby. 3 nights ago he started screaming when we walked into his room to put him to sleep. He's fighting naps and bed time hardcore. Sometimes up to 2 hours before he falls asleep for bed. This is so new to me. He's never made the slightest fuss. He's teething but he has already cut 6 teeth and he's never done this. And he's still so happy during the day. There hasn't been any change in his schedule. What's happening?!?