I have anal pain when and after I pass stool the pain is so great afterwards that it causes a sharp pain down my left leg

Something is pressing on one of the major nerves that go near the rectum and then down the leg. If it's the back of your leg, it's the sciatic nerve.
The reason for the pain behind the nerve could be anything - hernia is one possibility, but there are countless ones. You are going to have to be examined.