PR is a US Territory. Technically, you never left the country.
"In general, United States citizens and resident aliens who are bona fide residents of Puerto Rico during the entire tax year, which for most individuals is January 1 to December 31, are not required to file a U.S. federal income tax return if they have income only from sources within Puerto Rico." www.irs.gov/taxtopics/tc901.html
There are caveats. One that I know for sure is that if you are self-employed, you still must file & pay FICA taxes to the US federal government.