I have a fever, loose stools, fatigue for 2 days and back pain more than a week plus side of the body pain to flank area. Do I have disease/problem?

Yes. You may have a serious infection. Please seek immediate medical attention. .
See your doctor... Sounds like you are sick. To determine the actual illness, you should be examined (Flank pain is suspicious for a kidney infection; however, with the other symptoms it may also be related to intestinal inflammation from a stomach virus). Your doctor will be easily able to differentiate and treat you accordingly.