I think we've probably all used the cat scratch excuse. One time I told my boss, who had inquired about the huge bandages on my arm from an ER trip, that I had been washing dishes in really soapy water and got "bitten" by a knife I couldn't see. He bought it.
You said yourself why you do it, why most if not all of us do it: It's because nobody else can see our emotional pain. If you can make a physical manifestation of it, you can at least hope that people will see and try to help. Oftentimes it just freaks them out and scares them away, though.
I hope you can get some help to stop before it gets too far out of hand. Keep posting and let us know if we can help.
Candy