What Causes Brown Vaginal Discharge?

hi doc i'm 30 of age,5'2,not more than 100 lbs. i've been giving birth last year september 2011 and my doctor told me that she clean my uterus becaused there something in there she must be cleaned and she said to me that if i'm getting pregnant again that was the last, becaused my uterus now is very thin.. doc what i ask you now is that i missed my period for 3 months now and i felt sometimes dizziness, and i have this brown spotting. doc what do you think is the caused?? now i think i'm getting loose weigth becaused my in law said to me my body getting tiny.