Spironolactone is primarily used as a diuretic - it lowers blood pressure by reducing the overall volume of liquid in your vascular system.
Caffiene (which is in coffee) is also a diuretic but to a weaker degree. If you're feeling lightheaded, it may be that your blood pressure is dropping too low - the solution is to either stop the coffee, or the better one is to start drinking a lot more water. (Or if you get really desperate - drink decaf...)
The chances are that your body will adjust, but keep the water intake up as a precaution.