I have severe lower back cramping after a bowel movement & eating. it radiates to lower front stomach & down left leg. what could be the cause?

Speculation: With the limited info, here is one (amongst many) possibilities: a lumbar or sacral nerve root could be inflamed (for various reasons). Raising intra-abdominal pressure when defecating causes pressure on the nerve, increasing pain messages traveling ultimately to your brain. Pelvic disease (not nerve disease) may do likewise.

Constipation: It could be constipation or a number of other things. I would start by adding more fiber like metamucil to your diet for a week as well as two extra glasses of water and see how you feel in 10 days.

Pelvic girdle : Unifying dynamic explanatory for gastrointestinal dysfunction & radiation of pain to leg & groin is pelvic girdle. This bony ring has three joints & two, sacroiliac joints (sij), are largest joints in axial spine. Gastrointestinal dysfunction & sciatica are, imo, not uncommon symptoms concomitant with sij dysfunction (hypermobility), which can result from a variety of causes (see my other posts).