When i run or stand up for a long period of time i feel like pain in my left abdomen, what could it be?

This is a stitch: This is a type of muscle strain. In the running community is called a stitch in your side this is muscle pain and it's important that you be appropriately hydrated and reconstituted to help decrease this. With time and with exercise this will improve. You need to be evaluated by a physician to be certain there is not another cause.