“If my girlfriend had a ring and told me that I could pawn it and her parents are now trying to say that I stole it, can they have me arrested?”

Larceny is the trespassory taking and carrying away of the personal property of another with the intent to permanently deprive. The way you describe it, it seems like you were simply a mindless pawn. If you had any knowledge that there was something wrong with the deal, you probably committed a crime. You had better hope that your story and your girlfriends story match up, or else it is possible that you have been an accessory to a crime, which, depending on the value of the item, could land you in jail.