CM1984

Recently, everytime my phone charge says its complete, the charge goes down one seemingly right after i go to start using it. Also just tonight, i charged it, unplugged from the wall, and began to play a song on my phone, and about a minute into the song, my phone just shut down and so i had to turn it back on. Does anyone know what could be wrong and what i could do to maybe fix the problem? Any help on this matter would be greatly appreciated. BTW in case you are wondering, I have had the phone since July of 2011.