Im going throught the same situation...now at 38 weeks. I asked my midwife and she told me it could be the prenatal vitamins, the magnesium. As long as there is no blood in the stool..
I can't tell you if it's normal or not but I can somewhat relate. Usually I would get diarrhoea when I was on my period and since being pregnant I still get it (and crankiness) when I would've been on my period. It's so weird. Like my body is still going through the cycle without the actual period part. I hope it stops soon for you!