If smptoms of a sprain aren't getting better but worse. When should you see your doctor? And what could it be if it has gotten worse?

Not sprained. It is possible that it is more then a sprain. It could be a hairline fracture or some other complication. Get it checked please.
Sprains can be mild. Moderate or severe. So even with so called negative x-rays there is still significant injury and treatment can even range to surgery for a sprain which means ligament injury not bump or bruise. Small unnoticed but significant or koint cartolage fracture or a tendon or nerve injury are other possibility if u have been told its "just a sprain ".
Sprain . First, you need to make sure that your diagnoses is correct ( visit orthopedic surgeon) . You could have a tiny fracture which is not visible on x-ray. Second, try to avoid walking if you do not have special boot. Bottom line: see orthopedic surgeon.