Yes. Post cast pain is usually due to weakness from long term immobilization and responds to physical therapy.
Ask your doctor. The ankle may be a little weak after being in the cast. You also did not state why you were in it, and for how long. It is not uncommon to have some residual discomfort. However, ask the doctor. You should not be afraid to ask your doctor. Better safe than sorry.
Yes. It is to be expected.
Fibula fracture. Yes if the fracture is recent pain is common. Just make sure the cast is not too tight which can also cause pain.