How do I tell my mom

Last year I was on the shot, well it made me gain weight, I was getting way too much estrogen I was sick my boobs hurt so bad, it hurt to put a bra on I got these lumps in my breast from it. I got off had to wait to get on the pill and it happened again I stopped taking them she doesn't know I don't want her to think I'm trying to get pregnant because it does cause pain. I'm supposed to "start" them again here soon. I want to tell her idk how. I don't want on any form. (Btw I'm 18)