I am assuming that she is intact? (meaning she has not been spayed)
If so, when was she in heat last?
Ok, that helps things a lot. That rules out a more serious problem called pyometra. The discharge likely indicates a vaginitis, usually a bacterial infection, however it could also indicate a urinary tract infection that has caused the tissues near the urethra to become infected.
She does need to have this looked at by a veterinarian, because even if it is NOT a urinary tract infection, it can become one quickly. As long as she is not having an extreme amount of discharge or discomfort, this can most likley wait unil morning instead of seeking emergency medical care.
There is a RARE cause of this called a "stump pyometra" ---this is infection that occurs in the remains of the uterus/vagina after a spay, but the first things listed have to be ruled out first before reaching this diagnosis. It is rare - i have only seen 3 cases of this in 9 years of practice.
Let me know if I can be of further help.