The neck area tends to be extra sensitive and tender, along with the fact that most of our neck stubble swirls in different directions. This makes the neck a prime candidate for irritation.

The initial irritation was probably due to lack of technique, as in not keeping your angle constant. I would also bet that you are not letting your neck heal adequately before the next shave, which just further irritates the same spots. If this is the case it is best to give your face a rest for a few days before attempting to shave again.