Hi Brockenspectre,
Cramp can indeed be a sign of a lack of salt (or a number of other minerals) but that is not such a common cause for it in the UK with its moderate climate.
Lack of salt in the blood (hyponatriemia) is can be the result of taking certain medication or in hot climes excessive perspiration or excess drinking of non isotonic fluids.
I would echo the advice given already to go and see you local healthcare professional as they can tell you what the most likely cause is and exclude the few more potential serious ones quickly.
They can also do a quick bloodtest to see if you are indeed lacking salt.
HTH
FD