bulky uterus itself will not cause bowel disturbances and pain. But if associated with intestinal adhesions pain can occur.
According to the symptoms given by you I am thinking that your wife may be having irritable bowel syndrome
Irritable bowel syndrome, in this disorder the patient normally presents with cramping abdominal pain
One of the symptom is constipation. The patient suffering with this experience difficulty in passing stools, pain or straining during bowel movements, not able to eliminate stools or only small amount can be eliminated sometimes as mucus.
The treatment mainly involves making dietary changes, avoiding stress. Laxatives and antispasmodics can be used to get relief from symptoms.
You have to consult physician regarding this, in case of adhesions you consult gynecologist.