I have two options (and own both). If you're bf'ing, you probably won't need to bring as much with you either.
So #1 - the Lassig backpack diaperbag. Lots of room in here & pretty good on pockets, including one that is waterproof for wet/dirty items. It also comes with a changing pad & a detachable insulated bottle holder. I've been using it on & off for DS (he's 2.5 yrs) and I think there would be plenty of room to add diapers & a change of clothes for the baby (blanket, etc). It also comes with straps that velcro onto the stroller & then the backpack can be taken on & off via the clips on the end of the straps.This one is in purple, but you can look around & find other colors (mine is brown): http://www.amazon.com/Lassig-Casual-Backpack-Purple-Butterfly/dp/B0030ICJ08/ref=as_li_tf_sw?linkCode=wsw&tag=bookblog03-20
Options #2 - the Ergo backpack. This is great, especially if you're planning on wearing your baby at all. It's made to use with the Ergo, but honestly I think you could use it with about any SSC. It's smaller, but I was really surprised at the amount of room that it has. I used it recently with DS while I was flying. The small one doesn't include changing pad, but I'll also include the link for the larger one that does. I initially also bought the larger one, but didn't think it was much better than the smaller in terms of space. The nice option is that it has velcro straps that allow you to either strap it onto your carrier or a stroller. Or you can wear the carrier in front & the back in back. There are some great You Tube videos showing it's versatility. It was kinda a fluke purchase, but I was really pleasantly surprised on how great it is.
There is also the mother of all diaperbag backpacks - Baby Sherpa & Okatots Travel Gear. They were both just too big for my liking, but will hold everything!