What Does Spotting And Lower Abdominal Pain Indicate?

i have been on the depo injection pill for 2 years and have never experienced bleeding or anything on it no symptoms nothing, however 2 weeks ago i noticed when i went to the toilet brownish blood in my underwear and i had cramps and pains in my lower abdomen 2 days later it turned red in colour but there is hardly anything its been really light like spotting not even half a tea spoon a day. its been 2 weeks n im still spotting i was wondering what this could be as everyone says its normal to bleed on it but ive been on it 2 years and never experienced anything the ains stopped after 5 days.