Do you take potassium and magnesium supplements? We tend to flush electrolytes out when drinking lots of water. Potassium deficiency can lead to fatigue and muscle weakness. Magnesium deficiency can lead to concentration problems, confusion, depression, muscle cramps and tremors.

Doesn't hurt to pop some extra.

Maybe a blood sample checkup at the doctors office? Your iron values, thyroid etc?

And don't forget to drink your water.