Depo shot

Hi! So I’m 17, and I’ve been on the depo shot (birth control shot) since I was 13. I get the shot every 3 months. I never had a period when i began taking the shot. When i started having sex, about a year ago, I noticed that if it’s closer to the end of the 3 months or about time for me to get my next shot, I will experience period symptoms. Back hurts, slight cramps, but I never bleed. Except if I have sex, there may be a slight discomfort and then i May bleed during intercourse. I wanted to know if I was the only one or if anyone else experiences this?