Can anyone please tell me the signs of postpartum depression?

I had my baby 2 weeks ago and though I don't think there's a problem, my husband has been convinced for several days that I may be depressed. I just let it go so there isn't an argument or anything but all I can think is I had a baby 4 months early that will be in the nicu for a few months 40 minutes from where we live, trying to keep up with pumping, getting 3-5 hours of sleep a day since I had him, I'm just exhausted. (Sorry for the partial rant at the end).