Is it normal procedure to lower flaps at high altitudes?
What you saw and felt was the normal operation of the Speed Brakes.

Speed Brakes can also be called Spoilers. They are panels on the top of the wing which open up to create  drag and reduce lift. Some of the panels are also used for roll control.

They are often used in flight when ATC requires a steep descent or an immeadiate reduction in airspeed. They are also used in an Emergency Descent in case of cabin pressure loss. During landings they automatically deploy to increase wheel brake effectiveness.