Does anyone know when Depo takes effect? My son is 7 weeks and I never had a period before getting…

Does anyone know when Depo takes effect? My son is 7 weeks and I never had a period before getting it at my 6 week pp check. I was on it on and off for 16 years before this baby but last time I got it after a baby was born was 9 years ago so I can't remember when im safe! Its been 10 days since I got it