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It's hard to say what's going on, of course, since I can't see her. Dogs will often rub their bellies if there is an allergic reaction to a bug bite, bee sting or other allergen (grass, dust, etc). If you suspect this, you can probably use a little benadryl. Most vets recommend 1 mg per lb of dog every 8 hours.
I would also be concerned, though, that you are seeing a pain response. Some dogs will rub their belly like this to relieve discomfort. If that's the case, keep an eye on him until your vet opens and then get him in there. A twisted stomach can be pretty dangerous. Even simple constipation can cause stomach pain.
Let me know if you want to discuss this more.