Hi, Judy.

But then again, if it is 12 after I backwash, and it is 20 the next day, then something must be gunking it up, right?
Yep, almost couldn't be anything else. The water comes out fairly dirty when your backwash, doesn't it?

If so, your filter is simply doing it's job. It'll settle down and when your pool is clean, you'll backwash much less frequently.