Past 72 hours diarrhea, headache, body ache, no fever, extreme fatigue. Diarrhea is gone, but still feel exhausted w/ headache. What could I have?

GASTROENTERITIS. Acute onset of symptoms is most consistent with gastroenteritis. You need to rest, fast for 8-12 hrs then clear liquids as tolerated. You need full exam and lab screen/Stool culture to rule out infection. If symptoms persist abdominal US or CT to rule out appendicitis. If it is vital it will resolve in 24-72 hrs. If fever or pain worsens seek ER eval.
Viral. Sounds like a viral gastroenteritis but some of your symptoms are extreme. You do have features of volume depletion (fluid loss). Suggest a call. To your MD and get your blood pressure and labs checked. Talking oral fluids with electrolyte may help. Suggest see your MD.