Right now, you're in what meteorologists, ancient Greeks, and increasingly wilted houseplants call the "dog days of summer." It's not actually about dogs.
Where the name comes from
The phrase dates back to ancient Greece and Rome, who noticed that Sirius — the brightest star in the sky, nicknamed the "Dog Star" for its place in the constellation Canis Major — rises and sets alongside the sun for a stretch in mid-to-late summer. Romans called that window the dies caniculares, "days of the dog star," and figured the extra starlight was adding to the season's worst heat. It isn't; Sirius is 8.6 light-years away and couldn't warm a saucer of milk. The real culprit is Earth's tilt, not the sky. But the name stuck, and by tradition the dog days run from around July 3 through August 11 — which puts today squarely in the middle of them.
Your cat did not get the memo about "dog" days
Whatever the calendar calls this stretch, your cat is the one dealing with it. Weeks of sustained heat is exactly the setup that lets a coat get away from you — loose undercoat that would normally brush out clean starts felting together in the humidity, especially in spots a cat can't easily reach or doesn't bother checking: behind the ears, the armpits, the base of the tail.
None of that shows up as one dramatic event. It's cumulative. A coat that looked fine in early July can be quietly working up to a problem by the time August's heat piles on top of it.
The two-minute fix
Since you're roughly at the halfway point of the dog days, treat this as a checkpoint, not an emergency. Run your hands — not just a brush — over the usual trouble spots. You're feeling for anything that doesn't move freely: a tug, a felted patch, a spot that resists your fingers. Loose fur is a brushing problem. Anything already knotting together is a mats-in-progress problem, and it's a lot easier to deal with now than in the last, worst weeks of summer.
If the coat's already ahead of you, a professional groomer can clear it out before the dog days finish the job. Sirius, as always, will take none of the blame.