Acidic poop ?

Brittany

So every time my 11 month old poops and we don’t catch it right on time and we change her 5-10 min after her butt is literally bright red ( almost like a burn ) . I treat it with diaper rash cream and healing ointment and it gets better after like a day or so . I switched from breast feeding to similac full time about a month ago and it seems that’s when it started . Has anyone ever experienced this and any ideas why it could be happening ?