Depression can start at anytime, and you can get depressed anytime after having a baby, even if it doesn't hit right away. I didn't even notice my depression until my first daughter was over 18 months old, and after I started to look at the last few years of my life, I realized that it had probably slowly started after she was born.
Is there someone you can talk to? I would start with that, if you can find a counselor or therapist you can meet with and start talking to about how you feel. It may help you just to talk through things and figure out where you want to go from here.
I think there is a link in the stickies with a checklist that gives a checklist of symptoms to help you get a better idea of what depression is and if you might have it.