Understanding that at times, a diagnosis over the internet is difficult, your description of, " red spots and blotches on the belly and possibly gums - this has me concerned.
I am very concerned that there may be a problem with the dog's clotting system, most notably the platelets.
Immune Mediated Thrombocytopenia is a possible cause that I am concerned about. In general terms, this means that the immune system is destroying the body's platelets. This is usually caused by either an infection, the immune system being out of control, or even cancer can do this.
When the platelets are not there, or too low, you can see signs such as you describe, with the medical term being, "ecchymosis." If you look at the gums in the mouth, red dots (called petechia) may also be there.
Here is a detailed link which may help: http://www.veterinarypartner.com/Content.plx?P=A&S=0&C=0&A=1412
As I said, yes - there may be other causes. Needing to be cautious, I would recommend that you bring your dog to the veterinarian as soon as possible for bloodwork.
Again - yes - this could be a simple skin infection, but over the internet I must be cautious when guiding you.
I hope this information helps and guides you!
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