I have same symptoms and can be caused by anal sphincter muscle spasm from straining or increased bowel movements. Dr. Has me use nifedipine ointment on the anal opening to relax the sphincter muscle. Others on this site have mentioned they use diltiazem ointment (which is a drug of the same class and mechanism). Nifedipine/lidocaine ointment is a compounded medication so needs to be made at a compounding pharmacy. Some university hospitals make it.  

Hello, Hattie. 

My j pouch is also because of cancer. A month or so ago I had sudden and severe pain, it struck like lightening, almost bringing me to my knees, and lasting several minutes. I feared it was cancer again. I researched and found the following information describing the pain. It eased my mind to know what it is, and having the all clear after my scope in December was a relief too. See below, taken from the Internet:

Rectal pain symptoms

The pain of proctalgia fugax is sudden and intense, usually lasting less than a minute. But in some rare cases, the spasm can go on for an hour. It is described as a sharp, stabbing, or cramp-like pain occurring at the anal opening. The pain can awaken you from a sound sleep. The attacks seem to occur in clusters, appearing daily for a while then disappearing for weeks or months.

The pain of levator ani syndrome is felt higher up inside the rectal passage and feels like a tight pressure. The sensation is made worse by sitting and improves with walking or standing. The pain usually lasts 20 minutes or longer and tends to reoccur at regular intervals.

Most haemorrhoids only cause a mild discomfort, but the pain can become severe if the haemorrhoid thromboses. This occurs when the blood in the haemorrhoid clots. The symptoms are an excruciating throbbing or stabbing pain that begins suddenly and can last for days.

An anal fissure causes a tearing or knife-like pain when it first occurs and turns into a dull ache that can last for hours. The tearing of the skin may also cause a small amount of bleeding. Each bowel movement irritates the injured skin producing a sharp pain. The pain can be so intense that many people try to delay having a bowel movement, which only causes a harder stool and more pain when it is passed. 

You might take this to your doctor and show him / her. It could help your doctor prescribe proper pain relief.  Good luck to you, Hattie.