My 13 Month Old Daughter, This Is The Second Time

My 13 month old daughter, this is the second time in 2 months she has had her stool smell like a chemical or some sort. It's very very strong and the smell literally stains her pants. She is now on milk, we switched her from formula about 3 months ago. She's a healthy eater, no known allergies, and is very active. Her does poop everyday, just not a regular time. This also causes a horrible diaper rash that extends out to her outter thighs. Please help!