What could be the knot on the ankle that bruises and goes away for few days ?

My neice has a knot on her ankle that bruises and then goes away for a few days, assumed it was nothing but it is now bothering her. So, I took her to her PCP and he sent her for x-rays and blood work and said that what he thinks it could be is serious and that was it and I am worried. I asked him numerous times what he meant by that and he told me and her mother that he couldnt say for sure just get the tests done asap. My neice doesnt go back to see her PCP until fri. or possibly mon. Can you help me or give me an idea of where to look? What are the possibilities of what this could be/