Why is blood coming from my dogs penis?

Most likely, the dog is coming into season - meaning ready to be bred. Female dogs will display a swollen vulva and vaginal discharge starting approximately 12 days prior to being ready to be bred. When they are ready, the discharge will change to a 'wheat' color. DANGER! If you don't want puppies, keep your girl inside and/or under constant supervision. Intact male dogs can and will find a way. Female dogs usually come into season 2x a year, approximately every 6 months. That's an average though - the cycle can be longer or shorter. The heat usually lasts about 3 weeks. The best way to avoid this is to spay her.