Sweve - I feel your pain. Recently I went through this exact same fear.
As others have said, colon cancer at your age would be extremely rare. The odds of you having CC at 23 are miniscule; almost non-existent. It does happen, but typically only in people who have a very strong genetic predisposition. I don't know if this will put you at ease, but you have a MUCH higher statistical chance of being in a fatal car crash then having colon cancer at 23.
Setting that aside, the symptoms you are describing are not really correlated with colon cancer. A five pound weight loss is not significant at all. I sometimes lose 2 to 3 pounds in a day, depending on how active I am, how much I have to drink, and what I've eaten. Also, only having a bowel movement once a day is really of no concern. I don't even know that once a day is "constipated." And, to the extent it is, constipation is caused far more often by other things, such as lack of fiber, dietary changes, IBS, etc. There have been some recent studies which strongly suggest that constipation isn't even really a sign of colon cancer.
As for the abdominal discomfort you are describing - that is definitely not cancer. Your colon does not have pain receptors. Ergo, tumors in your colon do not cause pain. Complications from CC can cause pain once the cancer has spread. But, you had a CT scan which would definitively rule this out. (CTs don't diagnose colon cancer, but if it had spread through you abdomen to the point it was causing pain the CT would show it). So again, this is not cause for concern.
The symptoms you are describing are far more likely to be caused by something relatively benign, such as IBS. I can tell you from firsthand experience - IBS can make you feel like something is really wrong, because it really disrupts your bowels. Constipation, diarrhea, pain, pressure, bloating, a feeling of incomplete evacuation, inconsistent stool, all can be caused by IBS.
You said this is your last year of college - ENJOY IT! Don't waste time worrying about something that's not there.