Muscle pain is much more widespread than you might think.
Most people know that the soreness they feel after doing either too much or unusual exercise, or too much manual work, comes from their muscles.
Lesser known reasons for muscle pain are fever, the flu, some diseases (e.g. many rheumatic conditions, Myofascial Pain Syndrome, shingles ), dehydration, a virus, a flu shot, surgery or taking medicine (e.g. a side effect of Simvastatin for cholesterol reduction).
Many people think that their muscles are inflamed when they feel muscle pain, but inflammation is characterized by swelling and redness in addition to pain.  Such inflammation can result when an injury to a muscle, tendon or another part of the body has occurred.
But the most common cause for muscle pain is small contracted fibers in a muscle or their surrounding fascia: so called myofascial trigger points (“myo” means “muscle”).
Often, when widespread muscle pain comes as a result of the above-mentioned reasons, it actually comes from trigger points. These sore points have been in your body beforehand, but were latent / sleeping.
They can get activated by a disease or an event and suddenly you feel severe muscle pain throughout your body.
The most popular home remedies for muscle pain are rest and heat. These measures usually help to reduce or eliminate the acute pain.
Unfortunately these remedies do not eliminate your trigger points. Rest and heat only bring a change of state: active trigger points change to a latent state. This means they stop sending out pain.
However, because these trigger points are still there, they are likely to be activated again by a minor, apparently innocuous movement: your pain comes and goes.
The Trigger Point Therapy Work / Clair Davies, 2.edition 2004 p.45