I can feel a lump in the side of my belly - could it be colon cancer?

Unlikely but see dr: Difficult to say likelihood without more history, but for a tumor to be palpable through the abdomen it usually would have to be fairly large, and likely would not be the only symptom if it were this big. You would have likely had other symptoms before this such as obstruction, constipation, weight loss, fatigue, blood in stools, anemia etc.

Go see your doctor: It could be many things. Someone has to look at it and perform some tests.