It sounds like a bowel obstruction. Just let nature take it's course. Keep an eye on her, and give her some mineral oil to help lubricate whatever it is she has eaten. If you see anything protruding from her hind end, try removing it. Just watch for signs of shock.
A bowel obstruction can be very serious and deadly if left without vet care.
Signs of an obstruction can be vomiting, refusal of food and/or water, vomiting after ingesting food or water, loose stools.
If it is a large obstruction it may need to be surgically removed. Do NOT assume anything without a vet consult. Some obstruction can pass by themselves. Some may pass and tear at the intestines along the way. Some may never pass and just kill the dog.
If poisoned or having an allergic reaction a vet would still be the best bet. If poison time is of essence. If an allergic reaction they can give something to make the dog more comfortable.
Call your local emergency vet clinic and describe the symptoms. They can often tell you signs to watch for or tell you to just come in due to certain symptoms.