What Causes Swelling In The Foot Post A Fall?

Hi, I am really wanting to ask this question so bad. I have a friend of mine that fell down the stairs and sort of twisted her heel/foot in a way and she went to consult a doctor and he had said to do x-rays and what so ever, so she did and when the results came in the doctor had said nothing was wrong, no injuries, then a month after that he had prescribed antibiotics to her and her foot/heel all of a sudden got really, really, really swollen and she could not even walk at all. We went to take her into emergency to see what we could do, he prescribed medication to her to get rid of the swelling and for her to some what walk, is there a way we can figure out the situation why her foot swelled all of a sudden?