What is a valid reasoning for pulling someone over?

I was sitting at a stop light on highway 65. I did not notice that there was a police officer behind me. I noticed a Holiday gas station up the way so I merged over and went into the gas station to go to the ATM. I then came right back out and got back onto highway 65. I went a couple miles then noticed a cop was behind me with his lights on. I pulled over. Officer walked up and asked for my DL and Proof of Insurance. I gave him this information and asked why he pulled me over. He responded with, why were you avoiding me? With a puzzled look I said I wasn't and asked what made him think this? He explained that he was behind me at such and such stop light and that he watched me pull into Holiday. He said he went up to the next light, stopped to radar some cars, then I came by. He then proceeded to pull me over for no other reason other than he thought I was avoiding him. He went and ran my DL and saw I had nothing on my record. Then came back to my window and asked, You sure you were not trying to avoid me? I repeated no, I had no idea you were even behind me at such and such stop light. He then said have a nice day and I proceeded to go to work. Is this a valid reason to pull me over? No, no ticket was handed out and nothing came of it but I feel I was being harassed for no reason at all.