Why does it hurt across the top of my ankle?

Not sure: You should see a podiatrist as there a many reasons/causes. They can do a thorough exam, history and take x-rays to determine the cause.

Not enough info: Your question is not specific enough and there re too many possibilities. Eg. Did you drop a box on it? Has it been there for years? Etc.

Many causes: Bone spurs, synovitis, osteochondral defect, arthritis just to name a few. Consult with podiatrist.