If your mom is a nurse in the ob, you better tell her right away. She'll figure it out pretty quick. You know, she probably is the one buying all the tampons/pads for the house...And she'll notice you aren't using your share! Also, when you are pregnant and trying to hide it, you get secretive and hide yourself away. She'll notice that too. And the boobs and the moodiness. Telling her before she has a chance to mull it all over in her mind will be easier than waiting and fearing her finding out.
A note is really impersonal. If you can, you should tell her to her face.
All your symptoms sound normal. You need to start taking vitamins and get yourself to the doctor and get a check up.
You are more than 3 weeks pregnant. You count by the first day of the last period you had.
Good luck!