What is a flat single rash on lower belly that doesnt itch and appeared suddenly! can stress be the cause? or the fabrics or what sickness can cause a single sudden redness?

There are too many possible causes of such a rash to even guess. Stress doesn't cause skin rashes. See a doctor, preferably a dermatologist.

It is impossible to diagnose a rash without actually seeing it. There are a large number of possibilities. If the rash does not go away promptly, I would recommend seeing a dermatologist or your primary care doctor. Good luck.