Could Diarrhea, Weakness And Lack Of Appetite/weight Indicate Sign Of Parasite?

Hi,

My symptoms began 2-3 weeks ago, I first thought I had the flu, I had nausea, diarrhea x 2days and weakness, no fever, no vomiting. I also had nervousness and was jittery inside. Went to the doctor and was told I have the stomach flu. I layed in bed for 5 days and could not eat. I was treated for dehydration in the hospital. I have had CBC, BMP, Sed rate, Bilirubin, CMP, urine tested and everything was within normal limits. Went to a different doctor for a second opinion and he thought it was mono. That turned out negative. I have been losing weight very rapidly. I have no appetite and am making myself eat to try and prevent losing more weight. I have been sick for two weeks and have lost 19 pounds during this time. The last 3 days I have had more energy and was actually able to work although I do not have an appetite and am still losing weight, sometimes I have chest tightness. Any idea what this could be? I am now waiting for the stool O&P and culture to come back. Could I have a parasite? Any ideas of what could be wrong with me?