Can a subcontractor send me a bill for claimed work after I purchased my new home from the general contractor?

Question Details:

Purchased newly built home from the main contractor about 3 months ago. Now I receive a bill in the mail from one of the subcontractors claiming that I owe them for "Extras: New Home." I never agree to these changes nor was there ever a change order for these "extras." There were other changes orders from this subcontractor during the construction of the home, but nothing for this. They are also claiming expenses for work we did ourselves. Do I have any obligation to pay this? and can they put a lien on my property for it?

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