Well, I'm not a doctor, but I did get a B+ in Anatomy & Physiology. My guess is that those screws and plates are causing the pain. Considering the amount of walking you do, I'll bet the next shot won't do squat for your pain. Is that what the doctor(s) said?
Metal doesn't flex and is hard than bone, thus can cause problems if they stay in the body. Ask for another doctor's opinion on why they should keep the metal in your ankle and how long they want to keep it in there. If they have a plan to remove it at a certain stage or time, then that'll make sense of upping your pain pills until they are ready to remove the metal. Then of course, you'll probably have to rest it until it fully heals if they remove the metal.