To my opinion, you are too young to start depo shot; therefore, I strongly suggest to always consult your O&G.
Next, there are many reasons causing the light bleeding, including infections (STDs and others). Therefore, you can consult an O&G to visually check your vagina, the neck of your uterus
, perform an ultrasound
to identify your true problem.
Meanwhile, I would suggest to have protected sex always, I usually advise my patients to use condoms.
Other reason could be trauma from the sex; in rare cases, when "heavy sex" is applied; such problems raise in vagina (bleeding).
Hope it helped!