Vet at FirstVet answers:
It is not normal for a bitch to get diarrhoea when in heat.
The most common change witnessed in a bitch in heat is a change in behaviour. It is possible that bitches that normally have a sensitive stomach (i.e. when they eat something other than their normal diet, at times of stress or when changing diet) can more easily get digestive issues in conjunction with heat.
Diarrhoea and/or vomiting when the bitch is in heat can be signs of pyometra – an infected uterus. If the bitch gets digestive problems when in heat and doesn’t seem well, it is important to check that the vaginal discharge appears normal.