Maybe you're changing your focal point much more frequently while cycling in a way that you don't while driving. Try not looking at close objects like your bike computer or gears, and then quickly looking far ahead, and then quickly close again, and then far ahead, etc.. You could be straining/fatiguing your focal muscles. There's nothing on your bike computer that matters, unless of course you have a power meter

If that doesn't make a difference, go see an ophthalmologist.