Blood from the genitals can be "in season" signs, cystitis or even kidney problems. Some diseases also cause blood to be passed in the urine. Best for your vet to examine the dog incase the penicillin is not the cure. By the way - the inappropriate use of antibiotics (as in this case where a dog is injected with the drug) is illadvised. Unless the person injecting the drug is doing so under the strict instruction of a veterinary surgeon it will be contravening the Act governing the use of Scheduled Drugs. Penicillin is a schedule 4 drug that requires a prescription made on the diagnosis of a disease.