Rear camera and flashlight on iPhone 5s not working

Rear camera and flashlight on iPhone 5s not working. I get a "The iPhone needs to cool down before you can use the flash" message in camera app along with a black screen even when phone is cool, though the front-facing camera works/shows image/takes photos. Restarting the phone doesn't help. It's iOS 11.3.1, with a capacity of 16 GB. I have no idea if this is a hardware or software issue. This phone is a little over 6 months old, since it was a new phone replacement for my original iPhone 5s that had an unfixable battery issue. Please help. Thanks.