hello, Depot shot is very much effective in preventing unwanted pregnancy and to be taken once in 3 months. It has failure rate less than 0.3 per HWY. But there has several side effects like irregular period including long time amenorrhoea, prolonged infertility after discontinuation,wt change,headache etc. Need some investigation like USG lower abdomen and hormonal profile to rule out underlying pathology if any. Consult with your doctor and need to change to other contraceptive method. Avoid stress,take healthy diet,do regular light exercise and maintain personal hygiene. Be well.