Awkward bruise

So friday night i get home from work at about 10:30-11:00ishh... im super tired from a crazy busy night soo when i get home i change rightt into my jammies. Let me explain something first. I moved ohhh 4 and a half years ago. My room is at the front of the house and is the main window for the house. Sooo any kind of air gets into my room very easily cause the insulation is not that good... over the years my dad has just been adding and adding the insulation underneath my room in the basement but it just doesnt help... so as you can tell my room in the winter is usually freezingg! back to my story though. i was changing into my pjs.... putting my tank top on then trying to find a long sleeve shirt on to go over it... putting my shorts on then trying to find some longer pants to put on. well i found my pants first so i quickly put them on because i was freezing. i found a shirt and i went into the bathroom so i could put the long sleeve shirt on over my tank top then put my hair up wash my face and all that fun stuff. but anyways i start putting my shirt on when out of the corner of my eye i see this odly shaped bruise on the bottom side of my left arm..... it was just plain old awkward because it was the most ranom spot everrrr and i have no idea how i got it!..?