How to know if it's postpartum depression?

I'm 3 weeks postpartum today. I asked my doctor how to know if it was postpartum depression or just regular sadness and she told me not to worry unless I felt like I couldn't take care of the baby. I can take care of the baby, sometimes it's hard but I do it. So that doesn't apply to me, but I'm miserable. I've been crying at least 4 times a day, usually more, for a while now. I cry because of my body, over things with my fiance, I cry when the baby cries a lot or won't eat right. Sometimes I even get frustrated with the baby when she won't eat or when I'm feeling particularly bad about my body. I don't sleep well even when the baby sleeps because I just think and make myself sad. Sometimes I'm okay, though. Sometimes I laugh with the baby and I don't feel bad. Is it just hormones and insecurities or does it sound like something more? Should I bring it up to my doctor or just suck it up?