Don't know why I'm feeling like this

I gave birth to a healthy baby girl last year in April she's making a year....but since her birth i never went back to feeling myself you know that feeling where you're down and not feeling yourself but you do bounce back i never got that feeling that i bounced back and that I'm in control....i do feel happy sometimes but not for long that sad feeling hits me out of no where , i start crying for nothing , keep arguing with my bf and just keep feeling stressed. He is trying his best to make me happy but all i get myself to do is just feel this down. And scared to loose him.....is something wrong or is something happening to me?