Hi BethF89,
Thank you for posting here in Apple Support Communities. We understand that your iPhone's flashlight has gone dim, even after sliding it's brightness all the way up. To help with what's going on, we'd like to start off by asking you a few questions.
Did your iPhone go into Low Power Mode at or around the same time the flashlight went dim? Use Low Power Mode to save battery life on your iPhone
If Low Power Mode isn't enabled, can you tell us if you received any temperature warnings such as the one shown here: Keeping iPhone, iPad, and iPod touch within acceptable operating temperatures
How to use the flashlight on your iPhone, iPad, or iPod touch
Cheers!