What would cause someone to all of a sudden have problems staying hydrated to the point it effects the bp?

Multiple causes: Sudden dehydration may be sign of a variety of illnesses, in order of frequency: excessive exercise, allergic reactions, internal hemorrhage, use of diuretics, heart diseases (complete heart block, eisenmenger's syndrome), days after a major surgery, and a rare poisonous like snake bite. With your history of heart disease start by seeing your cardiologist as soon as possible.