What Causes Vaginal Spotting While On Depo Shot?

hi im on the depo shot and i was wondering if i can be pregnant if im due to go back for the shot between the 6-20th and i had sex like all the time with my husband and i still havnt taken the second shot yet. i was spotting a month and all a sudden it stop came back for a day and havnt spot yet. but i notice theres like redish like brown in my underwear,its not discharge it seems like dried up blood,what could it be/ thank you