"Shaking Like Mad" is most assurable adding in a whole lot of potential Oxygen. Combine that with even a bit of iron in the tap water and yeah, you'd get thin negs. Mix it again that way and you'll probably get thin negs again.
Stirring fairly vigorously with an small paddle *without making too many bubbles* its the best way, not too hot. Oh and try distilled or carbon filtered next time.