Don here from Shopify. :)
You can check in your settings > checkout menu in your admin to see which, if any, Google tags might have been added there.
I would check out the additional scripts area of this menu as this is often where this would have been added.
You should be able to view or edit the tag in place for this store in that menu.
Let us know if you can't locate or update this or if you need a hand with anything else.
How are things going with your business otherwise?