For 3 weeks now, i've noticed that my stool is watery instead of solid. i am not sure what it is. the first week i thought it was a stomach bug.

Several reasons: Your doctor will need to ask questions about your diet, dietary habits, recent trips, medications, sexual practices, and family history. Based on your answers, the doctor will be able to determine your diagnosis, or else what other tests need to be done since the possibilities are many- infectious (parasitic, bacterial), or non- infectious (drug effect, autoimmune, etc).