Imagine you're planning a party and expecting about 20 guests. Three or four friends told you they'd come early to help you out. But they don't show, and instead of 20 guests, you get 100. You're overwhelmed.
That's what's happening with our pain signals. The cells send too many pain messages (party guests), up to five times as many as in a healthy person. That can turn simple things like mild pressure or even an itch into pain.
When those pain signals reach the brain, they're processed by something called serotonin. However, we don't have enough serotonin (the friends who didn't show up to help), leaving the brain overwhelmed.
This is why we have pain in tissues that show no sign of damage. It's not imagined pain; it's misinterpreted sensation that the nerves and brain turn into actual pain. Because when your brain says something hurts, it hurts.
Other substances in the patient's brain amplify a host of other signals—essentially, "turning up the volume" of everything your senses detect. That can include light, noise, and odour on top of pain, and it leads to sensory overload. This can cause confusion, fear, anxiety, and panic attacks.