I am a real newbie here with the pool and trying to learn as much as I can. It was our first pool opening this weekend and have a question on the behavior of the pump. When shutting off the filter, water is rushing back into the pool through the skimmer closest to the pump. The water is rushing in hard enough to pop off the plastic top in the cement and flow over the cement deck. The filter was running fine and was shut on / off serveral times without issue before it started happening and was running at normal pressue when it happened, but now it is happening every time the filter is shut off. Any suggestions on what is wrong?