Diagnosed with stomach flu three days ago fever 99.1 no vomiting diarrhea only in morning before meds so tired and weak how much longer will this last?

Flu?: may last 5-7 days. Stay hydrated and treat the diarrhea. Supportive care is key. See a doctor if worsening or persistent symptoms

Virus: Viral flu like infections don't respond to antibiotics as they are intracellular infections as opposed to bacterial infections which are outside our cells. They last about a week but are notorious for causing fatigue up to a week or more after the actual infection as the body is repairing damage which uses up energy- it's called postviral fatigue and requires rest and patience.