before answering this question we need to decipher the issue. first explain exactly what happens when you masturbate. be very detailed.. are you circumcised/uncircumcised?? do you use lubrication?? are you worry about someone catching you in the act while masturbating??? we need details...
maybe the stress of college life is taking a toll on your libido. If you're worried about upcoming tests or don't feel you're on schedule for handing in a paper, that's going to affect you. Do you consider yourself 'depressed' in any way? If all is well however, and you still aren't getting aroused after several days of sexual inactivity, that might indicate an underlying physical issue. Isn't there a clinic on campus offering some basic tests for little or no money?