I've had a few episodes where it turned out to be nothing.. but better safe than sorry.. I'm just a very cysty kind of person...
I can't stress enough the importance of monthly self-exams, I have a card that I keep in my shower as a reminder... (and have given them out as christmas gifts) Wait'll you start to experience the wonderful world of mamograms...
It sounds like you've been a good camper and do this every month, so in all honesty, it's probably nothing, if you gave yourself an exam last month and didn't find anything, and there's something there this month then it's been caught early... that's the key and that's why you do self exams - -for that early detection.
It's possible that it's just a bruise... you'd be amazed the things that will give you bruises....
You're doing the right thing by going to the doctor... Friday is a long way off, did you tell the nurse/receptionist whomever makes the appointments, that you did find a lump - some doctors will fit you in immediately... (though given your age, I don't think you are in a high risk group)
In the mean time-- take care of yourself... spoil yourself.. and try not to dwell on it..