Hey you're probably still in the phase of conditioning your knuckles or learning how to punch properly. I presume you're still starting out? Everyone goes through a similar phase at the start man. probably just a matter of wearing thicker gloves until your knuckles heal up as you've probably got a few microfractures there which is hurting when you apply pressure on the knuckle. even after a year of training a while back (and mind you I train in shinkyokushin karate which is a martial art based on bare knuckle fighting lol) i'd still get the occasion knuckle pain when sparring and hitting bones, then training everyday with the pain, hitting heavy bags with no gloves on etc. nowadays the only finger i hurt myself post tournies is my thumb, probably from all the bare thumb knuckle or joint to elbow impacts