If this is a "new" gunite/plaster pool you may have a problem with plaster dust coating the filter.

I have a DE filter so I don't know if this applies to sand too ... but when I first opened my new pool I had a similar experience where I had to backwash often. The first backwash dumped a lot of dirty water. After that, the next time I'd backwash the water would flow out clear and return the pressure back to normal for only a few days before I needed to backwashed again. After a coupkle of weeks of this disassembled the filter and found the filter grids covered with plaster dust. Teh backwashing was not removing this dust (paste). I was able to wash the palster off the grids with a garden hose. I never had that problem again after that.