I had a similar thing happen to me earlier this year after I accidentally let my DX run too long and got a low battery alarm, then shutdown. When I got home I charged it up overnight and it seemed fine the next morning.
However, I then started to notice the battery bar going down a lot faster than normal and like you I seemed to need to recharge it at least every 2 days, or even 1 day if I had been using 3G.
I tried a few things but what seemed to work for me was to charge it up overnight, take it off the charger in the morning, do a menu/setting/restart, then put it back on the charger all day till I got back from work. Then I took it off the charger, did a menu/settings/restart and read for a while (hour or 2). I then put it back on charge overnight, repeating the process....
After a few days doing this, my DXG seemed to be back to normal battery life. I only need to charge it every couple of weeks, maybe once a week if I've been using 3G a bit.
Another thing I've noticed with my DXG, from the start, was that if you just charge it until the green light goes on, I don't think it's really fully charged. It always seems better for me to leave it hooked up to the charger for a few more hours (or overnight) even after the green light just goes on.
If you cannot sort it out, give Amazon CS a call but maybe don't mention that you've replaced the battery (unless it was done by an approved Amazon repairer). Good luck with this.