Diarrhea pada minggu ke 5 kehamilan boleh jadi normal, especially kalau baru makan something yang tak agree dengan perut you, a few of my mum friends experienced the same during early pregnancy, sometimes sebab hormone changes juga. But since you're worried and it's your first baby, better get your doc to check lah, just to make sure everything's okay dan make sure you're staying hydrated properly, sebab pregnancy memang kena extra careful dengan these things.