Why do I have ankle pain when I stand up on my toes?

Tendonitis. Assuming there was no traumatic injury, you could have an achilles tendonitis if the pain is in the back of your heel. If the pain is in the arch then another major tendon could be inflamed.
Ankle pain. It could be a number of reasons, possibly tendinitis, posterior ankle impingement, or tight achilles tendon- get evaluated by a foot and ankle specialist before it gets worse.