ipod touch says that its not charging

My iPod touch (version 5.1.), which is about a year and a half old, says that its battery is dying. When I plug it in to my computer it continues to give me the warning message, but doesn't die. I tried different usb cabels, but the cabel doesn't seem to be the problem. The battery life has been getting shorter and shorter recently too. Also, the ipod is not showing up in iTunes anymore. I can't synch anything and I'm worried that if I reset it, I'll loose everything and it still wont' work.