Parents often have to face a lot of things in their lives that are totally beyond their control: the state of the general economy and how it affects their jobs, their bosses and their attitudes, their clients, taxes, spouses (whether present or not, or whether current or not), and other things. The fact that you didn’t say “she’s snappy and crabby to me because of X or Y or Z” means that she’s at least doing a good job of shielding you from her major concerns (outside of your upbringing), but that, at least, is something that she may feel that she can control and has to control pretty tightly so that it doesn’t get out of control. (You know how some kids can.) Obviously, if the rest of her plate is full, she can’t afford to have you get pregnant, get an STD, fail classes in school, join a gang, yada yada. So she may be attempting to control you too tightly, or at least too tightly for your comfort, and maybe more than is necessary.
She probably won’t want to tell you about some of the big things on her plate if she’s kept them from you this long. She doesn’t want you to take on her worry over things that she’s already having trouble with.
Show her that you’re on her side in small ways. Yeah, the note is nice, and the offer of various gifts probably won’t hurt, but you don’t need her to think that you’re trying to put something over on her or trying to cover something up. Definitely do tell her what’s going on in your life – at appropriate times, and not when she’s dealing with a million other things, or mad about something else. Just be clear with her what the rules are, and maybe (in a non-confrontational way, if you disagree with some of the rules) why the rules have to be ‘just so’, and then not only meet her expectations for you, but exceed them.
Take a look around the house right now, for example, and see something that needs doing, and then do that thing. Does the trash need to be taken out? Laundry done? Groceries put away? Dishwasher loaded or emptied? Living room straightened out or vacuumed? (You would not believe what a stress-reliever it is for a mother to come home after work, for example, to find the house picked up, your homework done, and the smell of a well-cooked dinner coming out of the kitchen. Or to wake up on a Saturday morning to the smell of coffee already made, a clean kitchen, and waffles.)
Get her on your side in small ways, and she’ll be more apt to take your side later when it’s a toss-up which way she’ll decide a thing. And if she knows that she doesn’t have to worry about you toeing the line, then she’ll be able to handle some of her other concerns better, too.