What cause the left, ball of your feet to hurt?

Lots of stuff: Could a broken bone, a strain, a skin problem. Would need more information and an exam. Please see your doctor.

Foot.pain: I can be a number of things such as a fracture, stress fracture, bone bruise, order Roma. A simple x-ray of your foot will eliminate most possibilities. You should see a podiatrist for further follow-up

Metatarsal pain: Can have many causes - sprain stress fracture mortons neuroma gout arthritis. Best to see a doctor for a physical examination and then you may need other tests such as X-ray and blood tests