Can having no car insurance be a pretext to a traffic stop?

I got pulled over, in Bedford county TN, by a sheriff ,from Rutherford county TN. He was in rutherford county and pulled a u-turn back into bedford county. When I asked him why he turned around, he told me because he patrols the area and said my tags came back as having no insurance. He ask for proof, which i didn't have, and came back up to my window and said he could smell marijuana. My girlfriend handed over three small roaches. Neither of us had smoked anything prior to the stop. He penalized me for no insurance and gave my girlfriend a ticket to appear in court for simple possession. After talking to another sheriff about the situation he said that having no car insurance isn't enough to warrant a traffic stop.