I had never planned on pursuing Materials Engineering. After graduating with a BS in Environmental Chemistry and working in the alternative energy industry for a few years, I grew to love Product Development and decided I wanted to go into Product Development/Product Engineering. I researched open job postings, bookmarked the postings that attracted me, and then I researched the degrees required for the job. Electrical, mechanical and materials engineering came up over and over - and I decided along with my chem UG degree, I would pursue materials eng. With my UG chem degree, I didn't think I would be a top applicant, but I've been accepted to one program so far! It's not uncommon for non-eng UG candidates to pursue engineering MS degrees.
My advice: search for job postings that attract you, and research the degree required for those jobs. It might help you figure out which direction to take.