That's pretty normal--your body just needs time to adjust to it. It took me months and months for the irregular bleeding to stop when I started the shot. If your spotting gets heavy or is really bothering you, you could always try another form of birth control. There are other oral contraceptives, Depo Provera (the shot), Ortho Evra (the patch), IUD's, and vaginal rings, to name a few. You just have to find the one that's right for you. For me and most of my friends, the patch is our weapon of choice. Easy as hell and has far fewer side effects than the other choices
Good luck, Cosmo Girl!