I would think the addition of Potassium, even in nitrate form, would potentially help with cramps, not cause them. Is it possible you may have been even slightly dehydrated for any reason on the day or day before the workout? Has this happened more than once?
One other theory might be that since nitrates reduce the oxygen cost of exercise and thus can support working out harder and/or for longer, you may have pushed yourself harder than you're used to?