Why do my knuckles hurt so bad from punching a punching bag?

I recently got an everlast punching bag for christmas. My mom bought me some "UFC Gloves" from kmart and they turned out too small so im forced to use some mystery material choppers (i think theyre deerskin but idk) to punch the bag. When i first got the punching bag i didnt use any gloves i just used my bare fist, soon my pinky knuckle had a shooting pain everytime i punched it. it was pretty annoying. anyone know what this is? i mean obviously the pain is from not using gloves but what exactly went on in my hand and how could i prevent this in the future?