Thanks for the query on ATD.
At 36 weeks pregnancy, when the foetus is near term, and possibly cephalic with head towards pelvis then it should invariably cause constipation due to pressure over the rectum. But diarrhoea and pressure in the vagina can be seen when the head of the enters the pelvis and trying to negotiate through the passage even in cases of preterm labour setting in.
In either case it is advisable that you visit your physician for a per vaginal examination and to assess the descent of the head and also the consistency and dilatation of cervix which shall give an idea a to whether you are progressing into labour or the diarrhea is probably an accompanied gastrointestinal infection .
If it's a labour setting in then it should be accompanied with frequent uterine contractions as well which should be increasing in intensity with every 10-15 minutes.
I hope I have answered your query in detail,
wishing you good health,
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