What Causes Brown Coloured Stools Along With Bad Odour In A Child?

My daughter is 2 1/2 and for the past three weeks has had stools that look like brownie batter and smell very chemically. She has been taking 4mg Montelukast since 12/15/14, 3mg Melatonin and a vitamin for over 2 months. She takes this nightly before bed. I m at a loss since she s had no diet changes and has been on these meds so long. Any advice?