A B737-700 OVER PRESSURIZED AT FL380 WITH NO INDICATION OF A SYSTEM MALFUNCTION ON ANY PANEL. SELECTING ALTERNATE CONTROLLER RETURN CONTROL.

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WE OBSERVED A SUDDEN CABIN PRESSURE SPIKE OR SERIOUS BUMP IN THE SYS. WE ALSO OBSERVED ABNORMALITIES IN THE PRESSURIZATION CTL PANEL LEADING US TO DON OUR MASKS AND ESTABLISH PROPER CREW COMS. I INITIALLY REQUESTED A RAPID DSCNT THROUGH ATC AND WAS TOLD 'ROGER; STAND BY.' WE INITIATED ACTION ON THAT REQUEST AND THEN WERE CLRED LOWER. IN LOOKING AGAIN AT THE PRESSURIZATION PANEL; WE BOTH WERE CONFUSED BY THE FACT THAT THE CABIN DIFFERENTIAL WAS RISING AND THE CABIN ALT WAS DECREASING (NOT INCREASING AS WOULD BE THE NORMAL EXPECTATION IN CABIN PRESSURIZATION LOSS SCENARIO) AT A VERY SWIFT RATE OF DIFFERENTIAL PRESSURE AND A CABIN DSCNT WAS ACTUALLY OCCURRING. IT BECAME CLR TO BOTH OF US DURING THIS OCCURRENCE THAT WE WERE ACTUALLY IN AN OVER PRESSURIZING STATUS; NOT A CABIN LOSS SCENARIO. THE CABIN CTLR WHILE IN THE AUTO MODE GAVE US NO INDICATION OF ANY FAULT; IE; AUTO FAIL LIGHT ILLUMINATION OTHER THAN WHAT WE UNDERSTOOD FROM OBSERVING CABIN RATE OF DSCNT NEARING 4000 FPM AND CABIN DIFFERENTIAL ROCKETING TOWARD THE RED LINE. AFTER SELECTING THE ALTERNATE MODE ON THE CTLR WE REGAINED NORMAL (ALT GREEN ILLUMINATED CORRECTLY) CABIN PRESSURIZATION CTL THEN NORMALIZED AND WE THEN LEVELED AT FL300 AND CONTINUED ZZZ WITH NO FURTHER ABNORMALITY. PERHAPS WE SHOULD CONSIDER; IN ADDITION TO THE MORE SERIOUS LOSS OF CABIN ALT EVENT AND THE NECESSITY OF DSNDING QUICKLY; ADDING INFO TO OUR QRH REGARDING THE REVERSE SITUATION WITH AN OVER PRESSURIZATION EVENT.