system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does my skin feel so sensitive and why do my muscles ache when I have a cold?
gpt
One of the main reasons that your body aches when you are sick, like with a cold is that your body's immmune system is producing plenty of anti-bodies.
These anti-bodies also produce histamines which typically dilates (widens) blood vessel near an infection, this allows for more of the body's defences to get at the infection.  There are histamine receptors in blood vessels that cause them to dilate. As these chemicals are released into your blood stream they can end up in your muscles or other body parts. Various body systems can have receptors to histamine that can then trigger a pain receptor. 
In addition to histamines there are biochemicals called cytokines  that are released when the body has an immune response that are also known to trigger a  biochemical pathway that can affect pain receptors. 

TL;DR The body's immune system sometimes trigger pain receptors or make them more sensitive when they release histamines.