My legs become numb to the point where i can not walk. it happens sometimes when i move quickly or carrie something very heavy. what can this be?

See a doctor ASAP: You need to get checked out for poor circulation caused by artery obstructions in either your legs or abdomen. Also, could be coming from a spinal narrowing. Have you had in changes in bowel or bladder control? Also, you need to get checked out for diabetes. Regardless, all of these conditions are serious and require aggressive treatment.