It would have been slightly cooler in Jun, with the possibility that there was a bit of rain that helped to cool the air. These past few days have been warm and clammy. Try taking a photo later this afternoon, perhaps of a flower in your garden. Hopefully, if we have a rainstorm to cool the temperature down, you can try photographing the same flower (if it is still in bloom), and compare the results. If the pics are replicated with the same focus issues, then I think heat haze is the answer to your issue.
