I used to be a vet tech. The amount of blood you explain is not to be alarmed about. It should stop on its own and not get worse. Watch the incision closely today. If the bleeding does not stop, or it gets worse, call your vet. Keep her CALM, as running and jumping will raise her blood pressure, causing more bleeding. Offer food and water. WALK her outside. No running or playing. You can give her things to chew on to keep her busy. Lots of close attention for her through tomorrow. Anything 'not right', like VAGINAL discharge, bloody urine, the incision looking red and or swollen, sutures torn out, CALL YOUR VET. I'm sure she will be fine. :)
I have never seen that happen from an incision. May be some residual blood that leaked out? If it continues or gets worse, I would definitely have it checked by your Vet.