You maybe have a low battery , even though you just replaced it. OR it could be a problem that most people have : Your battery is not secure. Here's a way to find out. Place the battery in the required place after unscrewing it. The put the back on and wínd it back in. If you push down towards the center of the back , and a Reset/Download sign comes up , then the holder of the battery is loose , and that means it could have been dropped , shaken , or just over used. I'd suggest getting a new tamagotchi , but any other suggestions , you should try first.
To prevent this from happening : When replacing a battery , don't be to rough. Don't EVER use a sharp object to pry it out of it's case. Try not to drop your tama to often , this makes everything ware out about 3% faster.
Tama62