Very severe rectal pain that spreads to left testicle and all groin area. what could be wrong?

Proctalgia Fugax: Pain associated with that area that can involve the perineum in a male that has no real association with swelling or bowel movement may be proctalgia or levator syndrome which is spasm of the pelvic floor. It can take a variety of appearances and the essence of diagnosis is one of exclusion. A simple exam can rule out infection or other primary rectal findings.

Need more info: Things that can cause severe rectal pain include abscess, thrombosed hemorrhoid, and anal fissure. More chronic rectal pain may be due to muscle spasm or nerve entrapment. Over-exercising, particularly bicycling, can cause this. Also, in men, prostatitis could cause this pain as well. You should be seen by a physician to have this further evaluated.