What Causes Internal Bruising After A Fall?

Hi i fell down the stairs yest morn (15) steps from top to bottom and have cuts and brusing, my right arm is sore and i got pains and needles down it twice now, feel a bit dizzy and slight headaches. went to gp today and was told no need for xray and its internal bruising. How can he tell that without looking at me? worried i done damaged to the head.