We opened our pool a few weeks ago ourselves. DE filter was manually cleaned. Everything was working fine. Today, we were vacuuming up all the maple helicopters, dirt, etc. I happened to remember that we had not opened the valve that controls water flow in the pipe that leads to our pool slide. We opened up, and the slide had water. I happened to look at the filter pressure shortly after, and the PSI was up 10 points. I assumed I needed to backwash since I had vacuumed. After backwashing it was still high. My husband turned off the valve that controls the piping to the slide, and the pressure dropped to normal. What could cause this?