It definitely sounds like your little one has an intolerance to something and most likely dairy/cows milk. After a bout of gastro my little ones have often become intolerant to milk and the aftermath of their poos is always acidic, burning poos with a smell that is unique to acidy poos, which those who have smelt it know what I am talking about, it's not your run of the mill ordinary bm smell.
Hopefully you'll get some answers from the GP today.
As an aside, I cannot recommend Daktozin enough if you're needing a topical cream to help relieve the symptoms. I am also in the process of trailing the new Sudo cream so hopefully will find out if that does the trick also.