Hello,
First of all I am sorry that you have this pain which prevents you from doing what you love - Dancing.
From your description it is hard to tell exactly why you have this pain, a possible cause though is "Plantar Fasciitis" which is an inflammation of the Plantar Fascia at the bottom of your foot.
In order to provide you with the best possible assistance and diagnosis, please answer these short questions.
1) Where exactly is the pain located in your arch, is it closer to the heel?
2) Do you also have some pain in the heel of the same foot?
3) Is there any redness or swelling visible?
4) Does the pain go away after resting?
5) Does it also hurt when you are not standing/walking/dancing?
6) Is it sensitive to the touch/massage it?
7) Do you have high arches or flat feet?
8) What kind of shoes you usually wear when you are not dancing?
Here are some suggestions for things you can do right now, regardless of your condition.
* Ice your heel/arches at least 4 times a day - You can roll a frozen bottle on the floor with your foot.
* Buy off the counter gel inserts that you can wear in your shoes (when not practicing ballet).
* Stretch your legs and feet before activity or before getting out of bed in the morning.
* Avoid wearing "cheap" shoes like flip flops - they provide no support for your arches.
* Stop dancing for a little while and see if there is improvement with the pain.
Again, if you can answer my questions above I will be able to help you further.
You can write to me directly if you prefer, my email is itaiw@yahoo.com
Good luck and feel better!