Hi, 1 month ago i played a hockey match and got an extremely hard hit on the inside of the top of my shin.?

The most likely cause is that the lump has developed as a result of the original haematoma (a type of solid bruise) I know that it seems ages since the injury but a month is still relatively soon and I would probably give it a little longer. Occasionally, these things can become infected so if it feels hot to touch, is very red etc, do get it checked out.