Just changed four fluorescent bulbs and now they are constantly flickering. Did I use the wrong bulbs?

No, the lamps are not installed correctly. Turn the switch off so there is no power at the fixture, remove the lamps and as you are taking them out notice the position of the pins. Rotate the lamp in your hand until the pins are straight up and down place your finger on the bottom pin and insert it into the holder as both ends of the lamp should enter the holders at the same time you may have to stretch some what. After you have finished try the lights again if doing the same thing re post and I'll try to help!