Why would a police officer shine a spotlight into my car while on the freeway?

I was driving on the freeway on my way home a couple nights ago and a cop car speeds up and drives beside me matching my speed. He then shines a spotlight from his car into mine for about 5-7 seconds and then speeds off ahead of me.

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It was quite odd and I've never even heard of that happening before. My friend thought that it might have been because of my tinted windows and he was somehow checking the level of tint I had. Any idea on why a police officer might do this?