if you are drinking too much water that will cause an electrolyte imbalance.  
i'm a big fan of drinking a lot of water yes, but there is such thing as drinking TOO MUCH.  i've always found it silly how guys who weight train in an A/C gym seem to think they should be gulping gallons of water.  you don't need all that water.   go run in 90 degree heat and 90% humidity like i do and then come back and tell me who needs water and who doesn't.