Thank you for the query.
As large intestine issues has been ruled out, other possible reasons of your symptoms are bacterial infection of intestines, small intestine
issues (like partial obstruction due to adhesions), pancreas
issues. Back pain
can be caused by urinary tract infection
which quite often associates loose stools.
I suggest you to have stool tests, stool culture test and urine analysis at first. If this tests will be negative, abdominal CT with oral contrast (enteroclysis) should be next.
You may also discuss with your doctor Rifaximine treatment.
Hope this will help.