What to do if i am a 20 year old male ive realized my whole left leg is getting smaller muscle wise?

Go see your doctor: Regardless if you're 20yo as stated or 39yo as listed, if you notice a size discrepancy in your muscles, go see your family doc for confirmation & referral for EMG/NCV +/- muscle biopsy. This testing, performed by your Neurologist or Physiatrist, will help pinpoint cause/location leading to atrophy. It's important to tell your doc how size discrepancy affects function eg falls/injuries etc.