So it turns out that I had actually dislocated it at the top knuckle. I've been walking around with a dislocated finger for nearly a month. No wonder it was still so painful. I showed it to my friend yesterday who is an A&E nurse and asked her what she thought the lump was on the knuckle. She had a close look and immediately pulled on my finger. Weird crunching feeling and the lump disappeared immediately. It's still hurting but loads less than before. She said it will hurt for a while as it will be so badly bruised but it should be fine in a week or so...
Maybe I should have followed your advice
@Cynic Goat and got it checked out earlier