This sounds very much like an electrolyte imbalance of some kind. What are you drinking, water or a sports drink? Do you think you are OVER-hydrating, leading to a dilution of body electrolytes? Muscle cramping, abdominal cramping, severe weakness, nausea and vomiting are signs of both hypokalemia (low potassium) and hyponatremia (low sodium.) Palpitations/ fast pulse are specifically associated with hypokalemia.