Why my LG Chocolate BL20 is getting switched off automatically?

I am using LG Chocolate from last one and a half years & have recently found that my phone gets switched off automatically. I am sure that There’s absolutely no problem with the battery, UI or apps of the phone, but I don’t even know what the reason is, it’s happening first time. If anyone faced this problem than please provide me with appropriate solution, I will be thankful to you.