Diarrhoea is not a side effect of heat, so it seems there is something else going on with her.
Have you changed her diet at all? Or could she have eaten anything that she isn't used to? That could be a factor.
However, it could be a virus or infection too. Is her tummy tender? Is she behaving differently in any other way?
Obviously dehydration is a risk so make sure she drinks plenty of water and to be honest, I would suggest she sees a vet. There are so many things that this could be, some serious, others not too bad at all, that I would hate to make a guess and risk her health.
It might be a good idea to think of getting her spayed soon actually, as this can prevent a lot of problems and also spayed dogs are said to live longer.
I really hope she feels better very soon.