If Molly were my dog, I would call the vet now. Any competent vet clinic should have an outgoing message directing you to an emergency number. Bloody urine is in now way normal after spaying. It may be something simple like a urinary tract infection, but the timing with the surgery is just too coincidental. At least talk to an er vet clinic on the phone to find out what symptoms you may need to be aware of that indicate a life-threatening emergency.
wish i had a answer , you should call the er they can tell you weather to bring her in or not, but i think if shes eating and acting ok than she be ok till tomorrow, but always better to be safe call the nerest animal hospital they can tell you .