How about killing two birds with one stone? Take her to a vet and have her checked out. If she's healthy, get her spayed and that will be the end of that problem. If she's sick, you'll have taken care of her properly.
In most times, it is normal and considered as a sign of submission when a dog wants you to rub her belly. This may mean that the dog is contented with you. The aggressiveness behavior could be due to anxiety.