Dog Found Dead With Blood Coming From Mouth Of A Dog..why?

Sounds like it may have been poisoned

Lots of things that could cause that. Heartworms, stroke, seizure, an illness, poison like said, injury bunch of things. You would have to have your vet do a necropsy (can't wait though) to find a definitive reason and if you have other dogs I might consider it in case it could have an effect on them too and if you had noticed any symptoms or anything earlier and maybe didn't realise that's what you were seeing if it happens again you will know what you are seeing or at least know it's serious. I'm very sorry about your baby.

Perhaps the dog was abnormal

It sounds like the dog may have got into something seasonally that
acts like an anticoagulant or has an internal clotting problem that's
getting worse as it gets older. I don't normally associate bleeding with
heat,  But it could be a factor. Where do you live?  
However, due the situation in which the animal was found there are numerous factors that could have taken place. The Columbia SC dog trainers that I know could recommend a better answer!

It may have been poisoned. But its hard to say why or how. Thats really sad... D,:

If blood is coming from any where for the head and it is dead then that means a car hit it in the head so it has brain damage so it will bleed from the mouth out the nose,mouth,eyes,and eyes so don't worry it got hit in the head and had a fast death.

Maybe it has been poisoned or it had a disease.

Poison if it was wrapped up in a bag then it was probably tortured and killed but thats really sad