iPhone camera hacked

It may have been on at some point. If the orange light is on, some app has access to the microphone. But, you've not been hacked. Simplest solution is often to just restart the device. You likely held down the voice memo option.

If you received the message in the lock of the screen, You may be by mistake swiping a little towards the camera and I see the green sign ,if you do not jailbreak your device, you don't have to worry.

Open the control center and you will see the recently app used your camera!

What kind of message did you receive?

Then there is nothing to worry about.

Nothing