Hello,
Excessive amount or duration of menstrual flow, at more or less regular intervals is called menorrhagia. Sometimes it is associated with polymenorrhea(Frequent flow, cycles of ≤21 days). Causes of menorrhagia are Hypothyroidism, Endometrial proliferation or hyperplasia like ovarian tumor and polycystic ovarian disease,endometrial polyps and neoplasia,endometriosis,fibroids and uterine sarcomas. But in your case it looks like it is due to the depo shots.
In my opinion you need to get your pelvic examination done from your gynecologist along with investigations like complete blood count,thyroid function tests,PAP smear and ultrasound uterus.Once cause of menorrhagia is identified,treatment can be planned.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
I would consider a different form of birth control. Many women have this problem or others that prevent them from being able to use the shot.