Extremely high daily data usage

We are only 6 days into our billing period and have received a notice that we are close to exceeding our data limit. The online daily meter shows that on one day we used 301 GB of data and the following day that we used 408 GB. We are already already at 87% for this months and still have 24 days left in the billing cycle. We normally use between 500 GB - 600 GB per month. Nothing has changed in how we use the internet or the number of users. What would cause data usage to spike like this?