Please have your ankle x-rayed as soon as possible to be sure that you don't have a hairline fracture . If there is a slight fracture, and you continue walking and putting pressure on it, you will make matters worse. In the meantime, stay off of it, and keep it elevated on soft pillows.
Even though you may not have fallen, you may have tripped, banged it or twisted your foot, creating the pain. Ibuprofen or tylenol will help relieve pain for now.