Hi, Anicholea. I experienced similar pain, mine was a terrible and sudden stabbing pain, always on the left side too. It was one episode over a week or so. I used a sitz basin with very warm water to ease the muscle and encourage good blood circulation to that small internal area. It never happened again. Here is some information I found on the internet during my experience.
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.