If tiredness and nausea would be from anemia, then anemia would be clearly seen on blood testing.
And, if anemia would be from gastrointestinal
bleeding, than blood would be found in the stool with stool blood test (not necessary that blood would be seen with a naked eye).
Tiredness and nausea and weight loss could be from cancer, but **colorectal cancer is extremely rare under age of 40. **Small intestinal lymphoma is also rare for your age.