I think probably you had taken Depot Shot for contraception
before leaving the hospital.
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 ,abdominal cramp
Do a blood test
for beta hcg to confirm/exclude pregnancy.If you get negative test,then you may need to do USG lower abdomen and some hormone tests to rule out other underlying pathology. Consult with your gynecologist regarding this & take suggestive measure.