Lactic Acid.
Lactic Acid
Muscles cramps occur when a muscle contracts and does not relax. This causes pain in one or sometimes multiple muscles.
By a muscle that fails to relax completely resulting in the muscle going into spasm.
Muscle loss. Have you not been eating?
Lactic acid
There are a number of causes of cramps. Some causes can be being dehydrated, taking shallow breaths, over stretching of a muscle or exercising for too long.
Muscle fatigue has a number of causes including ion imbalance within the muscle or nervous fatigue. And most importantly, from the accumulation of lactic acid in the muscle.
Weakness, Fatigue, Muscle cramps, Constipation, Abnormal heart rhythms (arrhythmias) are symptoms are hypokalemia.
Tetany
Lactic acid builds up and causes muscle burn.
Fibermyosis is a syndrome that causes chronic muscle pain and fatigue. Sometimes it can cause weakening muscle pain.
Magnesium deficiency can cause muscle cramping. Potassium deficiency can, as well. These are minerals, not vitamins.
Lactic acid. While it doesn't actually cause pain by itself (it causes muscle fatigue), a buildup of it can.