Watch this spot for a while. As long as there is only one spot, and it doesn't change ---I'm not sure I would be too concerned. He may have injuried himself there and have bled under the skin. If this is the case, the spot should disappear in time.
If more spots appear, be sure and take your pup to the vet. My vet noticed several red spots on my dog's stomach during a routine checkup. She did blood work, and, unfortunately, my dog had thrombocytopenia. This is a serious blood disorder (the platelet count becomes very low and the blood no longer clots as it should). This disorder needs aggressive treatment.
With only one spot, I wouldn't be too concerned - but be sure and keep an eye on his stomach. Let's hope this just goes away!
Carol Jean |