If you push the "off" button on the shower head for about 5 seconds, when you release same the flow/pressure is good for another 5 seconds, then back to a trickle. This defys any flow/pressure logic and I'm at a total loss.
It's perfectly logical. Any obstruction creates a pressure drop. However, for a pressure drop to exist, you must have flow. When you push the OFF button, even though no water is coming out, flow continues past the restriction, going lower and lower, until it has re-pressurized the line between the restriction and the OFF button, at which point it becomes zero flow. When you push the ON button, and open it again, the initial high pressure allows an initial high flow, but it quickly drops down as the restriction prevents enough water from going past it to keep the pressure and flow up.
Bottom line: You have picked up a restriction somewhere. Either the filter has undergone a sudden clog, or you've got something sudden caught in the faucet or other similar place.