How do i know if i have stonach or colon cancer. my stomach feels very odd. little diarea and nausa. i have these llumps all over my body for long tim?

See a doctor: Best to see a doctor for a physical examination & possible blood tests. Longstanding "lumps" on the body may likely just be benign fatty lipomas. Unable to tell if a person could have a cancer without a physical examination, maybe endoscopy & colonoscopy. But you also seem quite young to worry about cancers.

See a doctor: If you are having stomach complaints then seeing a doctor, being examined, and possibly having further testing or endoscopy can accurately determine if you have cancer or are at higher risk for stomach or colon cancer. Both stomach and colon cancer more commonly occur later in life. Gi bleeding, pain, anemia, vomiting, changes in stool character are some symptoms that may be caused by cancer.