I wouldn't worry too much. My two girls (one passed away last year) were like night & day. One slept all the time & the other loves to be awake & active. Hamsters have their own personalities, just like people do. If it was a dramatic change after usually being active, I'd worry, but otherwise not. Also, after Donna died, Prima slept more because she missed her sister a lot. Your new baby might just be missing the hamsters it used to share a cage with at the store. It'll get used to you soon, don't worry!