I would not use any type of heat at this time as there is sure to be some inflammation. My guess would be a torn achilles tendon as well given your description. It may be torn all the way still because there are several other muscles around the ankle that help to move in all directions (ant. tibialis, post. tibialis, peroneals, etc.) You need to go to a doctor and get it tested. You need to know exactly what you are dealing with before trying to manage it.