For the last 3 days ive been having diarrhea i also get so tired my legs hurt constantly and i get very sleepy? what can it be??

Likely viral : Although there may be a number of potential causes for diarrhea, fatigue, and muscle aches, the most likely diagnosis, in the absence of other warning signs, such as blood in the stool or additional symptoms, is a viral illness. You should expect your symptoms to begin resolving within 5-7 days. They may evolve to include vomiting or abdominal cramping, but rest and fluids should help. Good luck!

Possibly dehydration: You could be dehydrated but also suffering from a viral or bacterial infection. You could replenish your fluid by drinking Gatorade and taking loperamide 4mg by mouth for the first dose then 2 mg after each loose stool. Don't exceed 16 mg in a 24 hour period. But also see your doctor for a stool studies to help determine the cause of your diarrhea.