If this is garden phlox, it is best planted outside in the summer. Flowers will come and go during the summer, then it dies in the cold weather, and you can get more of them to plant next spring.
To add to the above, phlox are perennials, so they likely could come up again on their own next spring, as well, as long as you plant them in the ground this summer.
Flowers are very short lived so it is normal for them to shrivel up and die. I would water it every other day unless the soil is dried up by the time you go out to water it the next day.