I was also actually wondering if you are just drinking too much water also. Really ...most adult females on an average day only need about 1200mls and with exercise 2 litres. The amount you are having might result in low sodium and classic cramps. Try either cutting back...or horror of horrors...adding more salt to your diet. You might need to have your potassium levels checked also.
Troubke is when in treatment we assume it's that. Do you start the additional fluid when you have it?
Anyways...a GP can do thise blood tests. Best be done aroubd tbe same time as the cramps.