How likely is it that a 20 year old have stomach cancer?

Unlikely: One truism in medicine is we never say never. It would be quite rare & significant enough to be reported in peer reviewed journals.

Unlikely: But if you have symptoms which bother you enough or lasting for more than a few weeks, you should discuss this with your doctor.

Cancer fear: Anything is possible, though what you mention is improbable. Please see your doctor to put your mind at ease. If all clear, reflect on the possibility that you may have a health phobia. If so, a clinical psychologist can be helpful. Peace and good health.