If you over fill the filter with DE powder it will dump some out in the pool. The problem with backflushing the filter and adding fresh DE is that you can never know how much DE was lost during the backflush and how much to add, It's too easy to over or under fill.

That' why I always dissasemble my filter and completely clean/empty and refill whenever the filter shows signs of being clogged. Fortunatley, it's only two or three times a year (on a year round pool).

Looks like you will have to disassemble yours for inspection - but if you don't see anything wrong (torn) you probably just over filled it.