I am sorry to hear your girl is not well.
Can you please tell me
How old she is?
Is the stomach larger then normal or just hard?
Has she been vomiting?
Is she having any trouble breathing?
What color are her gums?
When did she last defeciate?
I may have more but please try to provide as much information as possible. Since I can not see her I must rely on information.
Is she eating and drinking?
When she went out this morning did she have a bowel movement?
Has she had any new foods
Any chance she ate something she should not have?
If her stomach is hard and she is not vomiting it could be that she is tightening the abdomen in response to pain.If she is unable to bend her head to get her food and you have to put it in her mouth this could be a sign of spinal or disc injury.
Read more here
Hard abdomen accompanied by pain could also indicate problem with the liver kidney or spleen.
If she had not had a bowel movement this could be constipation
As you can see the possibilities are endless, because she seems so painful I would suggest having her seen by a vet as soon as possible. It would be best to take her to the animal emergency if your vet is closed.
I hope your girl is ok, please reply back if you need additional assistance