Due to chronic pain, i take oxycodone prn. it allows me to do simple tasks that produce mild exertion but when exerted, i start feeling weak and numb in my legs and faint, causing me to fall. what could be causing this? thanks?

We would need a lot more info but, off hand, both dehydration and poor conditioning could cause this. You don't say what the source of your chronic pain is. There are many, more serious possibilities such as anemia. If you faint or feel lightheaded upon standing the decreased blood flow to the brain is called orthostatic hypotension and oxycodone will drop your bp aggravating this. See a doc now.