Mysterious Blues

Can someone provide me with a logical scientific explaination as to how i got the blue-black mark as shown below?




Cos all i know was that i woke up with that horrible bruise after an hour's nap! Dun go saying that i hit something because unless pillows can cause that, im SURE i didnt hit anything. Weird thing is that, it doesnt hurt too!! =S




(i know what u're thinking!! dun tell me...ahhhhhhhhh......!!! It's THAT THING isnt it!!??!)