Electric heat costs are pretty wild. I've heard friends quote some insane costs over the years, so with the rate hikes, $400 sounds possible.

It's worth noting that RIE gave some kind of financial credit to everyone that obscured the true cost of the bill recently. It shouldn't have made that much of a difference, but perhaps the credit and the warm days in fall can explain the difference.

Do you have neighbors you can compare with? are you a first floor hooked up to the water heater?

I also recommend against turning off the heat altogether in the evenings or certain rooms. Many leases list a minimum thermostat temperature like 55 to avoid risking frozen pipe damage.