periods and pregnancy after depo.

hey my names steph,
i am 18 years old, i went to the doctor back in may of 2011 to see why i had not had my period for over a month. the doctor did a pregnancy test, the test was negative. he then put me on the depo provera shot on the 26th May 2011. i was suppose to go back for the second shot in August but decided not to as my fiance and i would like to start a family soon.
my question is, that for the past week i have noticed brown spotting on my panties on 3 different occasions, it does not have any smell. i am still yet to see a period after 6 months. i have been having slight abdominal pain, and tenderness on the underside of my left breast. is this all side affects of the depo shot? is there a possibility of falling pregnant without a period? should i be worried?
any help would be muchly appreciated.