Why does my betta seem freaked out about something?

I've had my crowntail betta for several months now with no problem at all. But just today, I noticed that whenever he goes to the left side of his tank, he starts making jerky movements with his opercular membrane extended. Only on the left side of his tank though. When he swims somewhere else, he's perfectly fine. It's like he sees a threat or he is trying to intimidate something but there is nothing there. It's very strange. Any suggestions?