Welcome to Nikonians! Congratulations on your new D3100.
Camera batteries need several charge and discharge cycles before they reach maximum capacity.
Were you using Live View to capture the 40-50 images? The reason I ask, is Live View drinks batteries.
The intent is to foster a community of "real people" instead of mysterious web-personalities with anonymous screen names. Doing this helps to establish and maintain a safe, civil and friendly environment.
Thanks in advance. Good Luck and Enjoy your Nikons!