For me I was at an increased risk with a history of depression. After my son's birth, he experienced some serious health struggles and was readmitted to the hospital. Everyone told me he was fine and I was overreacting. When my worst fears were confirmed, it was like the bottom of my world shattered. I've heard that having a risky/difficult delivery/etc can leave you at a higher risk.
My second birth wasn't as stressful, but eventually it crept back in. For me, the second episode was easier to manage than the first. I've also heard, from my therapist, that you are at a much greater risk of ppd once you've already had an episode.
((hugs)) to you. Staying conscious of the signs is the best thing you can do right now, and you're doing it. Read, learn, and listen to what is happening inside you.