The rust coloured shadow wasn't created by the lorry at all. Had you looked overhead, you would have noticed a medium-sized spaceship from the planet Yee-Yowzah* closely following your car and the lorry the entire trip. It was the spaceship's shadow you were seeing.

Later that same night the Yee-Yowzahnis abducted you and took you to their mothership where... well, we won't speak of that. Just be thankful they erased all memory of the encounter other than that "rust coloured shadow" from your mind before returning you home.


*
"Yee-Yowzah" is not the real name of the planet from which the aliens come. It's actually the sound made by roughly 50% of abductees upon their first encounter with the aliens' "special probe". The sound made by the other 50% is "What the fuck!", but "the planet Yee-Yowzah" has a much nicer ring than "the planet What-the-Fuck".