Just a bit confused here as to where the lump is, on the knuckle or between the fingers.
However it does sound like a haematoma, a sudden bleed into the tissues and may take time to resolve and it may even persist as a lump for the rest of your life.
It is not a big deal.
We seem to be in a transitional period, between it persisting and resolving.
Take Ibuprofen for the pain and give it another couple of months and if you are still worried then see your GP with a suggestion of a referral to a hand surgeon.