Oh my! The scar could be from a number of things. It could be the result of an injury as a pup that has left a scar. It could be from an exploratory surgery where the vet had to go in and see what she may have swallowed, etc. It could also be from a surgery called a prophylactic gastropexy. A gastropexy is a type of surgery that basically “tacks” the stomach to the inner wall of the dog’s body to prevent bloat. Bloat usually occurs in deep chested, large dogs, so pitties are usually not affected. But, it can happen to any dog; just not as common in those that are not deep chested (Greyhounds) or large (Great Danes). I cannot imagine that would be what the scar is from, but there are some vets who will do it for any dog over a certain weight. If she wasn’t spayed, then I highly doubt that a gastropexy has been done.
I hope that you are planning on having her spayed. By having her spayed, you will obviously prevent unwanted pregnancies in a breed that is already WAY overbred and prevent future medical conditions due to being intact.
Make sure that while she is in heat, you keep a diaper or something on her. You think the blood spots are a mess now, wait until there is more blood. You don’t have a mess on your hands now. Each time that she goes into heat, it could be much worse. If she is over a year old, then this probably isn’t her first heat cycle. Plus with a diaper, if she happens to encounter an intact male, she will have a lesser change of becoming impregnated. Though because she is in heat, she may cause behaviorial issues when she is around other dogs. So keep a good eye on her and other dogs. You don’t want another dog fighting with her because she isn’t spayed.