How long have the flowers been out, Moped? In my experience, individual Phlox flowers only last several days, then dry up and drop off. A cluster can last a few weeks, before it starts to look too ratty to keep.

It could be that the flower itself has finished and it has been pollinated so the petals are no longer needed. As Tug says each flower is quite short lived.
Or it could be due to the heat wave we are experiencing
or a combination of them both.

It's new phlox bought only last week. It looks quite happy apart from the petals dropping. It is a cluster so possibly it's coming to the end as you say. Thank you all for your speedy answers.

This is one of the drawbacks of buying plants in full flower. You can know what you're getting but not how long its been flowering already..

Keep it watered and it'll be good next year especially if we don't have such a heatwave!