Im a 30 year old male. my serum sodium is 135 mmol/l. my random urine sodium is 85 mmol/l. my random urine osmolality is 910 mosmol/kg. my kidney function is in the normal range. my symptoms are muscle cramping and fatigue. could this be siadh?

First, that serum Na shouldn't be the cause of your symptom and I can say that with 100% certainty. However, urine osmolality of 910 mOsmol/kg is quite unusual unless you have been advised to restrict you fluid intake because of hyponatremia and low volume can be the cause of muscle cramp & fatigue. If you aren't fluid restricted and pseudohyponatremia is ruled out, SIADH could be the cause.