An underpowered pump wouldn’t change because of a water change as you mentioned above. Did you check to see if the tubing came off the pump and air is escaping? There are only 4 parts to the set up of a sponge filter. Pump, tubing, air stone and filter itself. 2 of those supply the air, the pump and the tubing. The problem has to be one or the other. As I said earlier, tubing could be crimped or getting cut off by the lid or something or partially came off the pump. Second thing is that the pump has a problem and may be time for a new one. It could have been coincidental that the pump had a problem during a water change. As @Dwayne Brownsaid above, get another piece of tubing and hook it to your pump and put the other end in the water to make sure you are getting air still. Double check all connections of the tubing as well. Another thing I thought of does your pump have a valve to change how much pressure comes out?