here's a link to a post from a Mayo Clinic doctor about
in this opinion, the guy says that most people use "nervous breakdown" to refer to severe depression coming on pretty quickly. i'm not sure i've ever felt depression come on quickly. if you do feel that way, please call a counselor to schedule an urgent appointment. if you are starting to feel down & it is affecting your life, maybe it would be a good idea to see your primary care doctor to see if something is causing it (hormones, anemia, etc.). if needed, your primary care doctor can probably refer you to a mental health professional.
I hope that helps -- and that you feel better. Know that you can post here anytime. The other members are really wonderful people & are very helpful.