My lower back is tender after a stomach viral sickness once i came back from dominican republic. i had various lab test and all negative.

Hard to know . . .: . . . without being seen. However, given your recent travel, one needs to consider either a persistent viral infection (such as viral hepatitis) or a parasitic infection of the bowels--sometimes these do not end up causing diarrhea. Since your symptoms persist, you should definitely consider being re-evaluated, which might include stool studies (cultures).