thew way you got it is a prime example. generally, it's from a blunt trauma that doesn't break the skin. around the chest or abdomen, you can get internal bruising without showing outer signs because your internal organs are softer than your muscles and offer less protection to your blood vessels. at least i'm pretty sure that's how it works.
The trauma you received did not break any blood vessels that is why it does not show. However, whoever is sparing with you needs to not hit you so hard. Talk to your master about not wanting to spar with this person. Sparing is for training your body to be able to restrain itself. If this person is not able to restrain themselves, then they do not need to be training. Why should you listen to me? We'll I have been trained in martial arts for 7 years and I am a 2nd level 3rd degree black belt as well as an instructor.
If hit in the right way with the right amount of force you can be bruised only internally. Infact some bruising can be so bad that they are only found in the morgue..after someone died from injuries that no one knew about. (sorry for that)