Possible PPD

How do you know if you have post partum depression? I had my baby 3months ago. But she was born at 27 weeks and sent 78 days in the nicu. I was good the first week she came home but the last week I've just not felt like myself. I've only left my house for doctors appointments and grocery shopping. My husband has been going out with his friends leaving me home alone. I just feeel like I need time to go out with my friends. Does that make sense? I love my little one more than anything but just feel lonely.