Diagnostic tests normal but severe symptoms

My daughter, 33, gave birth 6 weeks ago.  At the beginning of her pregnancy, she began having the symptoms and was diagnosed with UC.  It was very severe (she was hospitalized) and none of her doctor's thought that there was any chance of her carrying the child to full-term.  She did and even the UC resolved itself during her 6th month of pregnancy.  She has had no problems until two weeks ago when she stopped nursing.  The UC has returned but the stool samples are negative for any bacteria and her white blood count is normal.  Her doctor is puzzled.

The symptoms are a little different this time.  She has urgency with mucous and blood but very little stools for about 18 out of the 20 times that she rushes to the bathroom.  There is no fever but pain when she eliminates. She is very health conscious, excellent diet and exercise.
Are we missing something?