Why do i take magical damage after resting?

is it a form of punishment?

i admit to have being a bit naughty beforehand, but totally in character naughtiness.

You only heal a certain amount, depending your lvl and con I think? Anyway, if you have like 10/30 hp when you rest and your lvl 3-5 you're bound to lose some hp to indicate that you didn't heal fully on your rest.

No, this is not a punishment. Resting doesn't fully heal you, so to reflect that after resting all characters take a certain amount of damage to reflect how much health they should have.