I was always under the impression to not start feeding yogurt until age 1, and to stick with mostly fruit and vegi puree as well as some cereals for introducing food until the child is 1.
If you think it is the yogurt I would stop feeding it to him. Is the only new food he has gotten in week or so yogurt? It's possible that you might have to eliminate all of the foods you introduced recently until he gets older.
After you eliminate the foods, if the red circles are still there I'd mention it at a well child check (or sooner if more symptoms develop or the redness gets worse).