Yep, my DD gets it too. It is from not drying properly or from her vomiting or sweating and it getting in the folds of skin. I always make sure I wash and dry under there really well- there have been many tears shed over drying under there properly! I also put on a barrier cream after a bath at night and then clean it off in the day time when I put on some cornflour powder. I clean under there maybe 2-3 times a day as well as after a bath.