hello, this is a weird condition, i mean please dont let such thing happen or dont make her that angry. but ya, let analyse the condition. the skin over the knuckles is comparatively thin so it is easily getting bruised. and if the punch was hard enough, it might crack the bones. now if any bone in the body gets cracked, the area becomes swollen immediately,and the movement of that part becomes painful and difficult. but in case of hair line fracture, which is very minor crack, the symptoms might not get this much severe. to be on safe side, i suggest you to consult an orthopedician and get the hand x rayed, in order to eliminate the possibility of any possible crack. and by that time you can start over to some anti inflammatory and analgesics. this much would surely do. thanks regards.