Hi,
There are a number of different things that could be going on here. The most likely reason for the droplets of blood, is that she has a urinary tract infection and is squirting some bloody urine out. If this is the case, she would actually posture as if she is urinating outside, when the bloody drops appear. If the blood just seems to be dripping right out of the tip of her vulva (without her posturing to urinate), then it is not likely that this is her problem. If you think a UTI is the case, then she will need a trip into the vet tomorrow morning to have her urine checked and to be put on an antibiotic.
Another possibility is that she is coming into heat, because of an ovarian remnant. In some female dogs, if a spay is not done correctly a piece of the ovarian tissue can be left inside. This can secrete enough hormones to make the part of the uterus that is left, and the vagina have a normal heat reaction. This would result in diapedesis (the normal bleeding of a heat cycle--although sometimes lessened in intenisty) from the vulva. In some cases, this can also result in something called a stump pyometra. This is when the small pouch of uterus that is normally left after a spay becomes filled with infection. These dogs will usually drip a bloody/pus-like mixture from the vulva, will become febrile, lethargic and will usually lose their appetites. This is a medical emergency and often requires hospitalization, IV antibiotics and sometimes surgery.
Another possibility is that she has somehow sustained an injury to the area. Perhaps by hurting herself on a sharp branch or other object on the ground outside when she squatted to urinate.
I recommend that you start by taking a towel and gently applying some firm pressure to her vulva. If she does have a wound in the area, this will help it to stop bleeding. If the dripping of blood continues tonight, and she is not showing any signs of lethargy or vomiting, then you can wait until tomorrow to have her seen...but, I would not wait until Monday. She really should be evaluated for the pyometra and UTI as soon as possible. If the vet determines that these are not the case, then your regular vet can do bloodwork on Monday to determine if there may be an ovarian remnant left inisde of her from her spay. If this is the case, she will need another surgery to go in and remove this tissue.
I hope that this information is of help to you, and I wish you the best of luck with your dog. Please let me know if I can be of further assistance.

Dr. K