Why Was a Semi Truck Flashing Its Lights At Me?

I was on a three-lane highway in the middle lane, going about the speed limit. A semi truck came up behind and started flashing its lights at me, doing it at least twice. I didn't really understand; none of my lights were out, it was the middle of the day, and there wasn't any traffic around us. There also wasn't any restrictions on trucks being in the left lane, so they could have passed me. What were they trying to signal me for?