pump pressure increasing quickly

I have a 16x34 IG pool with a 1.5 superflo pump/SD80 sand filter/Ig20 SWG/Pentair 300ch heater.
This is my second year with the pool and everything has been good till recently when I noticed that
my pressure is increasing rather quickly. Last year we opened in June and closed in Oct and we maybe had to backwash
twice... and both times it was because i just felt like I had to.

Fast forward to this year... the pressure is normally at 16lbs. Recently we had two quick but intense rain storms. The first one the pool cover was on all day and the pressure went up from 16 to 21 in less than 24 hours... Ok, so I backwash and brought it down to 16. This was yesterday. Now I come home from work today and notice that the pressure us already up to 18. The only thing in common with the two days was the rain and the fact that the cover was on all day.
You would think that having the cover on all day would keep a lot of stuff out. The pool has been clear.


Any ideas? I dont want to have to backwash every 3 days.