The average matriculating non-urm at UMd has an mcat above 31 and gpa above 3.6-3.7. You're gpa and mcat are both certainly below average to get into Maryland as a state resident. Seeing as how you admit that you have 'the usual volunteer' stuff, is there something else about your application that stands out to such a degree so that you should be surprised you didn't get an interview?
Maryland isn't an "easy" state school to get into, but even if you were from a state with a slightly less competitive state school, I'm not sure you would have gotten an interview.
I'd try to make an appointment with the dean of admissions at UMd as soon as possible so you can start working on improving specific points of your application. Concerning the 3.3 gpa in CS, I think whether or not you need to improve this depends on what your core science grades are, especially during your junior and senior years. If the reason you made a 3.3 is because you made a few low grades in upper level CS classes but did really well(3.8+) in your bio, chem, and physics classes, then I think you may be ok gpa-wise. But if not, it might be that your gpa is holding you back. 4 or so more points on the mcat would probably help you as well. But you won't know for sure until you get some feedback from UMd.
UMd is also apparently pretty big on shadowing and lots of hands on clinical experience. My guess would be to really get involved in that kind of stuff while you aim for an improvement on the mcat. I think those two factors would offset a 3.3 gpa and move you into the competitive category at UMd. Good luck.