Checkered garter found WAY out of range

Last spring I found this tiny garter (less than a year old, about 9" long) while herping in northern MD by my house. I didn't think much of it until I checked his body condition over. He was super underweight and I didn't think he'd last another day without food. I didn't want him to starve so I brought him home and gave him a few pinky pieces, planning on releasing him the next morning. Unfortunately we had a late freeze overnight and I couldn't release him. It was like that for the next week or so. At this point I decided I would just keep him as he wouldn't have made it in the wild anyway. As he got older his markings drastically changed and he looked nothing like the other garters I kept him with. After a little research I found that without a doubt he is a checkered garter. He has very distinct brown checkers all down his back and just in general looks like a checkered. My one question is what in the world was a checkered garter snake doing in maryland?