How Do I Know if I Have Postpartum Depression?

I know the obvious answer will probably be to see my OB, but what are some symptoms? Does it only last for a few months after baby is born? My son is 11 months old but I feel like ever since I had him I’ve been so sad...of course I’m happy to have him, I love him more than ANYTHING and have no regrets, but I always feel so alone. I feel like nobody understands what I’m going through and I have nobody to go to for help/advice about anything. Any comments would be greatly appreciated!