I would shock the pool.

Chlorine is consumed by 1) the Sun and 2) organics in the water. While there may have been a little increase in the exposure to the Sun during these long days, I suspect the beginnings of algae is a more likely culprit.

It's a cheap fix, too. Put in enough Clorox to bring your Cl up around 25ppm, preferably in the evening, and test the next AM. If it has dropped noticeably (more than 2 or so ppm), that was the issue.

I'd bring it right back up to 25ppm tomorrow AM. That'll give you some residual while your away and protect against further algae development.