Why do i have pain in the backside of my shin when i run on a treadmill for a while?i started exercising a week ago and the pain is mostly left leg

Pain in shin: area takes a while for your body to adjust to running. Make sure you are not running up an incline and also make sure you are stretching before running. Another concern in supporting your arch if your left foot tends to roll into the ground or flat. Try running short while then walk for few minutes and then run again, that will get you in better shape to run without pain over time.

Varies: There are a number of potential causes including "shin splints" , an early stress reaction of the tibia, and even exercise induced compartment syndrome. Warming up, stretching, cool downs, and not running on a real high incline can help; also some runners have pronation malalignment of their foot and there are anti-pronation running shoes available. Consider being seen if it persists.