How Can Severe Cramps In The Legs Be Treated?

My name is Rob.. I m 53 yrs. I m 6 1 healthy weight ( 12 1/2 stone.) I use my exercise bike after work each night for approx. 40 minutes) My job requires that I walk and stand for almost 10 hours each day.. (6am- 5;30pm ) I work as a farm manager, of a huge pig farm in Ireland ,, there are lots of heaters, I would sweat a lot at work, , For years now, I have being getting severe leg cramps, ( almost faint.. blood pressure goes v low,, and I get very pale) when I lie/sit for any period of time.. Should I be taking a supplement ??