What is the cause of cramp and dull pain from my left buttocks down to my feet always?

Many possibilities: There are many possible causes for the pattern of pain you're describing. A careful history and thorough physical exam would be helpful in determining the cause. I would suggest you see your doc about this. The possible causes could be muscular (a fascial distortion or a pulled/strained muscle), neurological (a slipped disc in the low back, sciatica) or some combination of the two.

Sounds like...: Sciatica. When pressure is placed on the sciatic nerve, a patient may experience the above symptoms. Because sciatica may occur for many reasons (slipped disc, strain, spinal stenosis), your doctor can confirm the diagnosis with a history and physical examination and then perhaps order blood work or imaging studies (such as mri) to support the diagnosis and pinpoint the reason behind the pain.