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Deposhots provide protection against pregnancy only if taken regularly and each shot is effective for 3 months.After coming off the shots it can take few months for the hormonal levels to come back to normal levels and have regular menstrual cycles and ovulation.
It seems you are also experiencing these effects.Though pregnancy is possible if you did not repeat the shot but it is difficult to comment regarding that.
Get a Urine test done for pregnancy determination and visit a Gynaecologist for evaluation.
Take care!