It could well be the heat. This year I've particularly noticed that both of mine aren't interested in food when it's warmer. They seem to want to eat when it's cooler, ie before 7am or after 10pm and aren't interested in anything in the day, even something they would love as a treat. Also, my two are losing a lot of fur at the moment with the heat, so that could increase the chance of furballs. As long as they are weeing at least 1/2 times a day and going to the toilet okay, I'd try not to worry too much, as I suspect you will see an improvement when it's a bit cooler.