Battery issues?!

Yesterday i charged my ipod and i took it out when it hit 75%. But later i checked it went to 65% then to 50%. during this time i havent even used the ipod after i charged it to 75%. Is there a battery probelm with my ipod?.... i use an charger which i bought from amazon and when i got it in the mail i found out its a knock off brand; may the fake charger be the problem?

Thank for your help!!