I tripped on stairs and my leg went in between the stairs hit the bottom my shin pretty good. Sever swelling and pain no bruising. What could it be?

A Bruise. Wait a few days, the bruise might be on a deep layer next to the bone, and it takes a while for the blood to make it to the surface to see. The blood might show up not where it was stuck but lower due to gravity effects and it might not bruise at all, swelling alone can occur after blunt trauma.