Baby suddenly prefers one side

My baby boy is five months old and has suddenly started to prefer one side. It mostly occurs when he's very tired or very hungry, so it's not even all the time. It's so strange to me. I produce an equal amount of milk on both sides and the flow is also equal on both sides. I've been thinking maybe I'm holding him differently but I don't think I do. I've tried different positions but it doesn't help. It's not a big problem, it's only hard when his not preferred side is full and he insists on nursing from the more "empty" side. Any advise?