If you are US citizen/Resident ( Green Card ) then you are taxed on your world income. AS such you have to file a return to be able to exclude your foreign earned income ( up to the maximum allowed) from US taxes. Thus you have to file a US 1040 return and include a schedule-C covering your gross earnings and expenses from self-employment in Mexico.
Where in Mexico are you and are you taxed there ? Is this your first year abroad? When did you enter Mexico?