If it's just one knuckle you probably just strained it and need to rest it up for about one to two weeks for it having the best chance of healing. If it is at all serious you could maybe have pulled a tendon in your finger or something like that. However this is unlikely because other parts of your hand and even forearm would be showing signs.
In any case take care of it ignoring these type of injury's can lead to the injury turning chronic which can end up taking months or even years to heal ( I would know ) fully.