[targonrotts]

first question. are you sure it was vaginal discharge? did it have a smell to it?
first rule out, could it have been anal gland discharge.
if it is vaginal discharge-i take it she is about 1-2 years old by your description-is it dark brown or a tan straw brown color? does it have a smell to it? how is her attitude? sometimes the very beginning of a heat cycle(in my dogs this happens up to one week before the actual bleeding) you can see a brownish/tan gooey discharge that later turns into the usual heat blood discharge.
pyometria, which is what the above have described is more common in older females-over 5 years of age. but it can happen at any age.
if concerned, if a fever, vomiting,lethargy behavior please conatct your vet. they can perform a vaginal smear,blood work or /and xray to rule out pyometria vs early heat signs.