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?

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.

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 ".