Why is my iPod battery draining and freezing during gameplay?

The thing is that when I play a game it froze and when I talked to my friends the battery is only on 60% and it shuts off by it self and I don’t know why and it skips numbers like when it is on 100 percent it goes to 97 percent and it keeps on going to a different number and when I put it back on the charger it goes back up to 35 percent and it never did that before