What Causes Constipation And Mucus In Stools Of A Child?

my child(18months old) always cry in the night for last seven days. she is doing latrine after crying. The stool is normal but it comes out with little amount of gel like liquid (just like disyntryproblem). what the cause may be for this? kindly advice. Taking medicine Bio-life sachet (pre & probiotic) as per doctor advice but not helping