I'm not a US citizen (I studied there for over 10 years), and I'm currently living in my native south america. The company I used to work for in-house in the US now contracts with me, so that's been my full-time job since January 1, 2015. Should I file/pay income taxes in the US, my country, or both?
You only pay taxes in Brazil, unless you're physically present in the US when you're performing the work.
Income for personal services is sourced to where you perform the services, and the US only taxes non-residents for income sourced to the US.