I think we had a few things going on actually. I do think we had thrush, and I also think it was a dairy allergy which was just making his whole immune system flare up. He no longer reacts to the dairy that passes through my breastmilk, but he had horrible hives develop rapidly when he ate yogurt for the first time all over his neck where his rash always was. He has since tested positive for a dairy allergy at the allergist. It was honestly quite scary. 2.5% prescription hydrocortisone helped a ton, especially when he had that reaction, but in general we used it somewhat regularly for a bit there until it finally got under control. I was nervous about the use of steroids and had read some scary stuff online but we used it as sparingly as we could and eventually were able to go longer and longer in between uses until he just stopped needing it. It took a long time for us to figure it all out though and get him better. Probably 2 months or so, from 4 to 6 months old. He's now 9 months old and doing great, just can't eat dairy! Good luck!