Wassup?

Okay so when I'm in my blockchain wallet and update my wallet the value on my btc (right next to the ammount of btc you have) go up and down like crazy. If you update and then update one minute later it can decrease in value with 100$ and then go up 1 minute later when you update again, why is it like this? I'm getting scared that bitcoins are crashing when the value goes down like that.