A common side effect of Accutane is dry skin, dry mucous membranes etc.
This dryness results in an irritation of the skin thus causing the itchy skin. It may also exacerbate any underlying eczema thus causing a flare-up.
My advice would be to use a non-comedogenic moisturiser to help in the alleviation of the dryness and itch and a Vaseline ointment for the lips if necessary.
If symptoms persist, please consult your doctor regarding the dosage of the Accutane prescribed and other treatment options.