In a.y. 2017-18 opted for section 44ad. in a.y. 2018-19 no

"However the Budget 2016 provides that where an eligible assessee declares profit for any previous year under Sec 44AD and the elegible assessee has not declared income u/s 44AD in any of the five consecutive assessment years succeeding the assessment year in which the income was  declared u/s 44AD, , then he shall not be eligible to claim the benefit of declaring the income on presumptive basis u/s 44AD for five assessment years subsequent to the assessment year in which the profit has not been declared under sec 44AD.This limitation will take effect from F.Y. 2016-17"

Simple interpretation is the disallowance is for subsequent years.

So, in your case, it will not be disallowed for preceeding year 2017-18.