Hello, how many times a day do you open your bowels?
Is it very water/loose?
Any blood or mucus in the stool?
Any nausea or vomiting?
How long has this been a problem?
Looking back at the information you gave me before, then it is unlikley to be HIV. There are many causes of loose stools, and I would hope that your doctor has sent off some samples to be tested for other infections that cause gasteroenteritis and C. Diff.
If you have not had a colonoscopy then this would be the next investigation to look inside the lining of the intenstine for signs of inflammatory bowel disease. Although bleeding may simply be haemorrhoids, it could also suggest some inflammation inside the intestine.
It depends which 'arthritis test', inflammatory bowel disease may raise some inflammatory markers in the same way that arthritis will.
Rheumatoid arthritis may cause loose stools in some patients, however people with Crohns Disease may also have inflamed joints. So you definately need more investigations to see which is the predominant problem.