Sore muscles can be causes by
muscular stress/ overwork, inflammations,
muscular degenerations, systemic infections, a few autoimmune conditions, hormonal/ endocrine abnormalities, micronutrient deficiencies etc. Without an evaluation and probably a few tests, it would be difficult to point out a single cause. It is best advised to consult your primary care physician for an evaluation for the above mentioned conditions. After a cause is diagnosed, it can be managed accordingly.