Not suicidal, just curious?

I want to know if your happy then why do all of a sudden you get really really sad, it feels like the whole world is caving in on you and you dont know why, and you just want to cry and give up? Im happily married to a wonderful man, I have 3 wonderful kids, whats wrong with me? how do you pull yourself out of it? could my hysterectomy cause this, or am I losing it, please I really need some advice.