I have a problem with my Nikon D3100?

Its something with the memory. I only have 44 pictures on my memory card, and its 8 GB. I don't know what happened, but now, I can't take any more pictures.
Its hard to explain. Heres what I do.
First, I take a picture. Second, I try to playback the picture. (I press the play button).It doesn't let me see the picture. So I turn off the camera. I turn it back on, and the playback button doesn't work anymore. So I take out the battery, put it back in, and turn on the camera again. I press the playback button, and the picture is gone! I don't know what's wrong. Do you think its the camera or the memory card?

Did you remember to FORMAT your memory card before you used it the very first time? If you didn't… Then transfer all files from the card over to your computer, then format it via your camera Menu.

Just format your memory card using your cameras menu and it will work again. This happens once in awhile to photographers and they lose many photos. Make sure all your photos are saved to the computer and format the card, it will completely wipe out everything and make it work perfectly. Do this every time you put your pictures on the computer to prevent it from happening again in the future

I think Petra_au and Cam are probably right. Just to expand on it a little, did you ever copy a file to the card from your computer, or use your computer to edit a file on the card? That kind of thing could get the card's interaction with the camera confused. (Remedied by formatting the card on the camera as they suggested.)