If you got no significant drop in chlorine overnight, then I don't think gunk in the filter is the issue. From what I have seen in weather patterns most of NE USA and our area in Canada (Toronto) has had much greater than normal amounts of sunlight. We only got 1/4" of rain in June and normally get 2 1/2", so UV action on the chlorine will be much higher than normal. Maybe just carry on and see if it goes back to normal for you when we get more normal weather.

By the way, despite what some pool stores tell you, filter sand almost never needs replacing.

Good luck