Bleeding after years on depo injection no previous bleeding

Patient: I have been on the contraceptive depo injection for roughly 3 years now and during this time i have never had a single period or even any spotting. a few months ago i had a small amount of spotting after having sex, but by morning it was gone and i thought nothing of it and it hadnt happened since until 2 days ago. i have not stopped bleeding since then and even though it isnt much blood at all i still would like to know if this is normal?