Ladies and/or mothers out there, how do I...?

How do I tell my mom that I use tampons? When I suggested the idea just merely a year ago she flipped out on me and got really upset and said she didn't want me to use them. I'm 17 and graduated high school, but she still doesn't want me to use them (she'll go out and buy my pads). I actually started using tampon's about a couple months ago during a trip to Hershey Park with my friends; I found it much easier to use a tampax pearl and a panty liner/thin pad because it's not as noticeable as a thick pad and it's more comfortable and I'm not paranoid about any leaks. I don't want her to get mad at me about going behind her back and buying tampons, but at the same time I feel that I should be able to make a choice as simple as that instead of hiding tampons and going through so much trouble sneak and buy some. What should I do?