Bills are suddenly insane - how are they calculated?

To my knowledge, bills are calculated using your lot value, how much lighting you have in your house, how long each light is on, use of electronics (e.g thermostats) and your income.

In your case, check if you have a lot of lighting or if you have a lot of electronics (like for instance multiple thermostats). Other than that, it is definitely weird and could possibly be a glitch.