The way you can tell that your dog is in heat is when you can see blood being spotted that is coming from your dog. It's almost like a female menstrual cycle. This is the time when your dog can become pregnant, the scent of the blood attracts male dogs to mount the female dog.
I have a Catahoula, born on 9/18/08. She's been acting kinda punky, not very playful w/our other Aussie pup, just wants to lay around. Could she be old enough to be coming into heat? What age? Is this a common sign?