implantable contraceptives are having some side effects like,
-weigh gain: which may be responsible for your stretch marks
, as body mass or fat increases to accommodate that the skin will stretch leading to marks,
and mood changes: that may the cause of your tiredness and bad dreams,
there is around 1% failure rate for it, so there is chance of pregnancy
so you once visit gynecologist and get blood test
discuss your problem with her and follow her advice,
if you are having smoking habit try to quit as it will increase the side effects.