So... I have some bruises and I need GOOD excuses for them. They are on my cheek and my arm. Any ideas??
I dont really care what the real reason is but if your intent is to hide them, you're not doing a very good job. Just dont worry about it. If someone goes out of their way to ask you what happened, go out of your way to lie if you really dont want them to know. Although to be frank it sounds like you do want people to know, as you went out of your way to tell all of us about it.
Silly everyday things. "Oh. Me and a friend was playing Wii, and she was going to mime jabbing me with the elbow and failed and really hit me." "A friend got a longboard skateboard. I tried it. Wooosh!" "Dropped the shampoo bottel in the bathtub and slipped when bent over to get. Good thing it wansn't prison." "A Ypu know X big dog. We were playing around and the dog manage to run straight into me." "X and me slammed into each other in the kitchen door." And so on. Usually when you hurt yourself its in a silly way. Think back on real bruises you gotten over the years and see if you could adapt any of those stories. Dont try to hide it or avoid the subject. Just roll with it.
Don't be a child; face up to the real reason like a responsible young adult. If somebody is harming you, or you are harming yourself, then you need to find help immediately.
That's ideal, but people need to reach the point when they feel ready to do that and it might take time. We dont know anything about the situation, and judging her because she might be scared and at the moment feels like a good excuse is best way to handle the immediate situation is the right thing atm seems useless.
If someone gave you those bruises, forget excuses. Get yourself away from there, tell the truth, and press charges. In that order. From the locations you mentioned, I don't think you ran into a door.