Several years ago, a study was published in the journal Dermatology measuring hair loss (shedding- hair fall) in women over the course of 6 years.
They found that in summer, there were more hairs in the telogen phase (resting, getting ready to shed) and that those hairs tend to be shed from the scalp about 100 days after the middle of summer. Just figure about 3 months after your summer solstice - the longest day of the year.
That means if you notice more hairs shedding in late September, into October or November - it's normal. For folks in the Southern Hemisphere, your autumnal shed should be around late March and into April and May.
The hairs you shed are going to be replaced by new hairs.
Of course, this time of year comes a change in seasons for a lot of us and sometimes that also means drier air and more wind - and therefore hair that may tangle more, dehydrate more readily and need extra conditioner or lubricants like oil and if you don't keep up with that requirement - you can have extra shedding too from the extra friction.
But if you're seeing extra hairs when you detangle and you and your scalp are otherwise healthy and the shedding goes back to normal in late November or December - you were probably just experiencing the autumnal shed.