Male Dog Bleeding From His Private, Why Does That Happen?

Mop it up and keep singing!

Well, it seems to be that your dog is in HEAT. Heat is a growing up thing. It happens to women  too. ( It's not called heat) Your dog will have heat for 2 or more weeks. If it goes over, don't worry she'll be fine. Other than that the only thing you should be aware about is that during heat, she will be looking for a mate so I STRONGLY guaranty that you keep her away from male dogs. She might get pregnant if you do so. My dog just had hers and the only annoying part is that you need to clean the blood. I also recommend to keep her off of rug or carpets. It Will be a mess! I hope this helps! ( Ohh and don't get worried. It's VERY natural for a dog.