What explanation can be given as the cause of a muscular pain from my left thigh to the knee and affecting my waist causing severe and unbearable pain?

Difficult to say: Not all limb pain involves the bone or muscle. Nerves from the back travel down the leg, but rarely stop at the knee. Injury to the pelvis (falls) can travel down the leg to the knee, and cause spasms which affect the waist. If this is not the case, you must review if your abdomen is the problem - bleeding in bowel, urinary problems such as kidney stones and bleeding can cause severe pain too.