Green stool in past bm's. Today they were sort of flat. Colon cancer? Ibs? Great grandma had colon cancer at 70. Should I worry? No other symptoms.

Relax and remember. Most troubles can easily be investigated early. These symptoms are not likely predictors of a problem. The usual suggestion of trouble is blood mixed in the stool. A change in the bowel habit may also suggest a problem. If you feel concerned see your doctor to discuss having a colonoscopy performed.
Likely no problem,, The stool may be due to your diet more than anything else. Although there are exceptions, you're not nearly at the age that this presents and you've got no other symptoms like fatigue and weight loss. If you're still concerned, see your doctor and discuss the screening guidelines for colonoscopy and other tests. Best of luck.