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?