Don't worry about it. There is no vehicle importation permit to the U.S. unlike going the other way. I know friends who study in the U.S. in border regions and took their Mexican-plated vehicles with them and they encounter no problems despite being up there for many years. They also have copies of all their immigration paperwork with them and keep their U.S. insurance up-to-date. The point being, you should have something that shows you still have residency in Mexico, (FM2/FM3/Diplomat/imigrado papers) otherwise the police would indeed wonder why you are driving around a vehicle with Mexican plates. The registration papers from Mexico I assume would be in your name.