When did you start minoxidil? I am guessing that you are in your first or second months of minoxidil usage. That is a VERY common thing to happen for the first few months of minoxidil usage. For most of us, it goes away with time and continued Nizoral usage. It is completely normal, very predictable, and nothing to worry about despite it being a short term distraction.
If you've had it for 3 months or more, then perhaps you could try cycling in an alternate anti-dandruff shampoo such as T/Gel or T/Sal a couple days a week and see if it subsides.