It sounds like the DE passed through the sand filter and went back to the pool. I have a filter designed for DE and if the filter panels have any holes, what you describe is what happens: instead of the particles staying caught in the filter, they pass through the hole and get dumped back into the pool.
NOTE: I didn't know I needed to turnoff the valve to my robot the first time I added DE to my filter, and it jammed the filter for the robot and sent DE all over the bottom of my pool as the filter for the robot wasn't designed for DE. I originally thought my filter panels were bad.
FYI: I have no idea if DE can be used with sand filters. The DE filter panels I use have a very fine mesh to trap the DE. With DE covering the surface, it traps even smaller particles. However, it also restricts water flow, that is why there are so many panels with very large surface areas to allow enough flow. Does DE get trapped by the sand and then result in trapping even finer particles? Would it clog the sand filter and restrict flow such that its not really cycling; therefore, not cleaning much water? Will DE just pass straight through? I don't know.
Besides the pressure gauge, I can tell when my filter requires cleaning by looking at the water in front of the pool light at night. If I see a bunch of particles floating by in the water, its not working so good anymore. If all I really see is air bubbles, and no particles, its working great. Maybe your sane filter needs cleaning, or sand replacement, or some other form of maintenance? I know nothing of sand filters.