I have chronic pain on the back/outer edge of my heel after standing for long periods of time. what could be causing this?

Heel pain: At 19 one thought I would have is Severs Disese especially if you are active in sports. This is an overuse injury that cause damage to your heals bony structure at a microscopic level. Simple skin problem can cause pain or an achilles tendinitis.

Many possibilities: Knowing almost nothing about you makes this difficult. Less than optimum footwear, too much weight, poor posture, too extended time 'standing', circulation or under-developed leg (foot) muscles. A podiatric doctor can evaluate this for you perhaps. Try other, supportive footwear (if possible) to see if it helps. Best!