4 weeks ago i got my palatal expander put on. in the past few days, i've started to grind my teeth at night. why is this?

Teeth shifting: This may be the effects of your teeth shifting , the slightest movement of your teeth can create pressure on contacts and your bite seem a bit off. This is a slow process, but usually will correct itself as the palate moves. If the noticeable grinding does not correct itself after the braces are complete you may be in need of a night guard.

No More Status Quo: The muscles that control your bite have been interrupted by placing the teeth in a new position. You are trying to grind down the new interferences to get back to the original position. To help during the transition a temporary night or day guard can be used so you can grind on the appliance and not damage the teeth or cause muscle pain. This is a normal reaction to a new position.