My mother is bipolar so I was concerned about PPD before I even had the baby. I talked with my doctor and he prescribed me an anti-depressant (sorry I don't remember which one). There were only 1 or 2 days where I just couldn't make myself feel happy no matter what and so I used it on those days. What a difference it made. But just because you don't feel depressed doesn't mean you aren't experiencing ppd. I learned that when other people began telling me things I was doing or ways that I was behaving that were not part of my normal character. Unfortunately they waited for like 6 months to tell me so I didn't know or I might have done more to deal with it.
There is no shame in talking to your doctor, for having ppd or for taking medicine for it. And you shouldn't wait until it gets bad b/c bad could have you taking a big jump off the nearest cliff that you can't return from. PPD is no joke, so err on the side of caution for your baby, your family, and most importantly yourself.