my dog stomach has red blotches on her stomach....she is a puppy of 10 months currently taking flea /tick and heart worm medicine.
If there are biting flies at this time of year in your area, the abdomen area is a common place for them to bite on a dog as there is little fur there. Often their bites look like a red bruise or a blood splotch on the abdomen. Black flies, midges, gnats, no-see-ums all can cause this.
You might see if a dog safe bug spray from the pet supply store helps resolve this.
If not I'd see your vet for an in person look just in case its a tick carried disease despite her being on prevention.
Hope this helps you!