Filter Issues

Noticed something interesting about my filter this morning.
Normal operating pressure is 15psi on the gauge.
The pressure on it this morning was around 11psi.
I noticed this because I figured it might be time to backwash.
After backwashing my filter, the pressure came back up to 15psi.
Is this normal? I’ve never seen my filter pressure go down when it needed backwashing. I did notice that the water coming out of the waste was nice and cloudy white.
Any ideas?