Yeah.... that's not a good sign. From my experience, itchiness always accompanies a shed.
On the bad side, you might need to anticipate a shed.
On the good side, sheds usually come, and then go... with stabilization and an end to the itchiness on the back end.
Right now, I would try to avoid using anything inflammatory on your scalp. Check your hairspray and styling products... can you make an effort to minimize them? And in addition to the nizoral, you might want to introduce a second anti-dandruff component, such as Head and Shoulders shampoo and cycle that in every five days.