Can I get a Master's in Economics with a Bachelor's in Mathematics?
Mathematics is a good undergraduate major for a prospective economics grad student. And I suspect that you could gain admission to (and succeed in) a master's program somewhere with the background you already have. But graduate programs in economics do generally expect that you've taken at least some upper-division economics courses, so you would be a much stronger candidate for a master's program (or even a PhD program) with a bit more preparation.

For example, here's what UConn (where I earned my MA) has to say about application to graduate programs in economics http://econ.uconn.edu/grad/apply/faqs/:


  Do I need to be an economics major to apply?
  No, though some economics background is preferred. Students with little or no economics training, however, will usually only be considered for admission to the master’s program and, if admitted, will generally be required to take some undergraduate economics courses before beginning graduate study.


So, I think the answer to your question is "yes" for at least some values of "master's in economics". Completing intermediate level microeconomics and macroeconomics (or, if you can only take one, micro) would probably improve your chances of acceptance, though, not to mention giving you a better idea whether this is something you want to spend a year or two on.

From your description of your interests, though, it sounds to me as though a MPH is a better fit for you.