One of the side effects of stopping smoking is that you lose you "regularity". I believe it's something to do with muscle relaxing effects of Nicotine (an old girlfriend used to call the effect of the 1st fag on the bowels "The Zebs"). When you stop you also may experience some constipation. Rather than take laxatives try a fibre suppliment based on psyllium husk
Makes Bran look like ice cream and the best poo you'll ever have, helps with the detox too.