You could ask your GP (or maybe nurse, but I suspect GP will need to authorise) for a renal function blood test. That's the test which includes your sodium level. If you have a drop-in blood centre, get the form from the GP, and take it to the drop-in blood centre when you have cramp. You could also ask for full blood count (FBC) and liver function tests, which might show some other cause if it's not slow sodium.
Alternatively, try eating something salty, and see if it goes away.