Do you think it would be prudent of you to ask one of your ballet teachers about it. I know professional ballet dancers would likely be able to either sympathize or offer good advice because the achilles tendon is worked a lot in ballet, and if worked improperly, may leave to unfortunate consequences. If you let your teacher know that you have been stretching it, she will probably be glad to hear it from a medical and dancing perspective. So, if you feel comfortable bringing it up after class, your teacher may be a good person to go to.