My nipples are getting darker and im slightly spotting brownish blood and i have the marina my question is what is the possibility of me being preg?

Always possible: Mirena (levonorgestrel) is one of the most effective forms of birth control. Unfortunately, no birth control method, including Mirena (levonorgestrel) or tubal ligation is 100% perfect. Once Mirena (levonorgestrel) is properly placed, it is over 99% effective and works consistently to prevent pregnancy. Some women may experience irregular bleeding after Mirena (levonorgestrel) is placed. Do a pregnancy test if you are worried.