Indian tribes are sovereign entities. This means that they can make an enforce their own laws. The Supreme Court has limited tribes authority to prosecute non-members on the Reservation in Oliphant v. Suquamish Indian Tribe, 435 U.S. 191 (1978). Many tribes have cross -deputy agreements that allow tribal officers to make arrests under State law. You are likely being prosecuted by the State under State law. You will need to contact a lawyer to resolve this dispute. Good luck.
This is not intended as legal advice. It is only provided for educational purposes and cannot be relied upon as legal advice. Further no attorney client relationship is or has been formed by answering this question.