I had protected sex six days back with a girl whose HIV status I donot know. I just had loose stools once today. Could I have hiv?

Doubtful. You say you had protected sex - then you should not be at risk for contracting HIV. One episode of diarrhea 6 days later does not a symptom of infection make. If you are going to be this anxious about HIV status, you may need to rethink your sexual practices and partners and avoid that type of intimacy until you know them better...
NO HIV Risk. If you had protected sex then you are not at risk of HIV infection. Zero risk. Diarrhea is not in any way a sign or symptom of an early or new HIV infection.