Lots of things can cause the weight loss. Stress and eating habits are just a few. For me i just don't eat enough calories and have managed to bottom out around 120 ish.
Which is 30 lbs under what i should weigh. My doctors have run numerous scans, blood tests, and scopes but haven't found anything conclusive.
If you are having other symptoms i would go back and have them do a full physical if you haven't had one already. Then if nothing comes back you can chalk it up to college life and all that walking. Sometimes lifestyle change is all thats needed to treat something like this.