Recently, I've been stressed, tired and constipated. When I did poop the color is yellowish, but now its dark green. Is this a sign of stomach flu?

No. Many things cause constipation. Stress is one. Get lots of rest, drink lots of water, eat fruit and if needed get miralax (polyethylene glycol). It is safe and effective over several days. Stomach flu makes you feel sick. If you are feeling well stool color is not important unless you see blood. That requires immediate investigation. You may need stress management if you can't manage on your own. Read up on it.