What could cause green stool, fatigue, nausea, and burning in stomach?

Maybe an ulcer: Symptoms/signs as described may signal ulcer, but ulcers can be painless and don't necessary have to bleed. How serious the problem and what organ(s) is(are) involved will be determined by appropriate medical evaluation--see your doctor. In the meantime, if you're taking nsaid's (aleve, advil)--discontinue them now, since you expressed concern about an ulcer and these drugs may be a contributant.