Why did they do that? Were they calling you an asshole for speeding? Or did they think you were too close? Or other??
I've heard sometimes people will flash brights to warn of cops ahead but we were going in the same direction, so that doesn't make any sense. Besides, there was another driver who flew past even me, but he was in one of the far right lanes. So if there was a cop and the brights-flasher somehow saw him in the dark, he probably would have gotten that guy.
Related: What are your driving pet peeves?