It.now there is an extra bone right beneath my ankle on the inside of my foot that sticks out and this is absent on my other foot. what could be causing it?

See below: This problem mandates a real-time (face-to-face) meeting with a doctor. A history has to be taken along with a examination as well as labs and other tests. Only after all this will his doctor be in a position to tell him what's wrong and what needs to be done to help him.

Possibly...: From what you describe, the prominence is likely the navicular bone. If you feel from the ankle bone down to it, you'll likely feel a tendon. This is the posterior tibial tendon, which helps support the arch. Sometimes there is an "accessory" navicular present, which makes it even more prominent. If you are having pain, see a podiatrist. Otherwise, it is unlikely you need to be concerned.