I have not idea why it happens, but you are not to blame yourself or allow the dog to shift the blame to you because he thinks you may have cheated too. Fact is, he is the one who cheated and he alone bears the blame.
You may be able to rebuild your relationship after this blow, but please don't let your husband's betrayal make you lose sight of your real goals: Eating right, drinking plenty of liquids, taking all your vitamins, and exercising every day. Take care of number one first.
Best wishes,
Maria