Pretty much that! – Muscles going haywire is for some reason the most unimaginable pain I’ve had. Why’s that?
In: Biology
The short answer: Ischemia
Muscle cramps (or spasms) are involuntary muscle contractions. The muscle (or parts of it) contracts way too tightly, resulting in inadequate blood supply to the muscle. This results in not enough oxygen being delivered to the muscle cells (ischemia). This lowers the pH in the area, causing a release of several substances from the cells which stimulate pain receptors.
Fortunately, muscle cells can survive ischemia for quote long (several hours) in contrast to brain tissue for instance (few minutes).
Edit: So as to your question why muscle cramps are so particularly painful, it’s because ischemic pain are some of the most intense (which from an evolutionary standpoint makes sense, since ischemia leads to irreversible damage of not relieved fast enough). A comparison is a heart attack/blood clot in vessels supplying the heart. This also leads to ischemia, and hurts like hell.
There are exceptions though, like a stroke where you have ischemia of an area in the brain. But there are no pain receptors inside the brain, so you don’t feel the ischemia here, even though irreversible brain damage occurs after a few minutes.
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