I am extremely fatigued. i had a bit of nausea and diarrhea last week, i rested this weekend but i feel flat. i've been taking echineaforce and vitamin c but still feel lousy.

Doctor can examine: Normal, healthy people do not have "constant tiredness," without a good reason. One's primary care doctor can follow along and look for reasons for fatigue, such as anemia, infection, auto-immune disease, thyroid problems, cancer, insomnia, etc... Various tests may be needed, depending on one's whole story, and sequence of symptoms.