Is it legal for an unmarked policeman to follow you for 5+ miles and then pull you over in Portland, OR?

I was driving to work one morning (6am) when I noticed a Dodge Charger starting to follow me as I was getting on the freeway. I changed lanes to see if it would do the same, and it did. Then we came upon a slight down hill and I sped up to try and pull away from the car. At first I thought it had worked because the Charger remained at the same speed, but then it sped up after me. I thought if it could of been a policeman, he would pull me over but he did not. We came upon heavy traffic and he still followed me, and closer now. After a little while I decided to try and make a move to lose him. I saw a good opportunity and took it. Starting in the left lane, of three, I crossed into the far right lane, sped up ping a few cars, and then switched back to the left lane. It worked. The Charger was stuck a car behind me. Then I saw a flash of light two cars behind me. The car behind me moved and the unmarked policeman turned his lights. Is there a law that restricts policeman from this?