Dear Jon,
Most US medical schools require that you have an undegraduate degree from a US college. I would assume this is not the case for you. In addition, non-citizens are not eligible generally for US loans (medical school in the US is not free and can cost you upwards of $200K USD) so many schools require you have evidence that you can pay all 4 years of medical school tuition and living expenses.
Therefore, your best approach is to finish medical school in Portugal, although you could certainly apply to some US schools and see what your options are. There is a book called the MSAR (Medical School Admission Requirements) available from the AAMC (American Assoc of Medical Colleges) which will give you specific details about how many non-US citizens schools take, and what the requirements are. It can be purchased on-line.
If you cannot get into a US school, consider schools in the UK or other EU countries, or finally coming to the US for residency training. Of course, it depends on where you want to work. Check the Portugal requirements to work as a physician - you don't want to train in the US, if you can never come home to work. Medical degrees are not necessarily transferrable across countries.
Finally, the English phrase is "my hands are TIED" (not tight).