If you are off depot shot few months back already and have not resumed your menses yet or have missed menses in this month, then it is always better to rule out pregnancy with a simple urine pregnancy test . If its negative then chances are that what you are experiencing is a part of premenstrual syndrome and indicate that your menses would ensue in a week.
It is not possible for you to bleed if you are already pregnant unless you are undergoing an abortion or have a threatened abortion bleed during pregnancy. So , in either case, pregnancy has to be ruled out before any further decision can be taken pertaining to your symptomatology.
I hope i have answered your query in detail,
wishing you good health,
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