
Depending on how long after the mating this is it could be the uterus clearing out it's contents.
A clear smelly discharge could of course be her emptying her anal glands due to stress excitement etc.
I don't understand why you can't get near her to check, unless you mean you can't bear to be near her???
If the discharge is definately from her Vulva, then I agree a trip to the Vet is needed.