Could this possibly be something else, or something in combo with thrush?
The rash in the neck could actually be a strep that could be cultured by
a dermatologist. Treatment would be different than for thrush.
Again, this is my experience as a mom -- one of my boys had this
condition at 4 months of age. The redness in the neck (not really a
rash, but  a shiny redness) DID culture positive for strep. Treatment
wasn't drastic though--the MD wasn't overly concerned bec. the numbers
weren't that high. We treated it topically. Interestingly enough, this
boy also had a white blister in his mouth that I treated with gentian
violet for a couple of days. It disappeared, and I never had
symptoms-EXCEPT for about three months earlier, when he was three weeks
old-I had EXCRUCIATING pain upon latch on,  but no cracks, blisters,
scabs, nada. The pain let up after let down. Only in retrospect did I
wonder if this was yeast, the pain was exquisite but did not last more
than a few seconds (mind you, the pain started at 3 weeks, not at birth,
and went away just as suddenly as it came!)Truthfully, I always thought
of it as a "nerve pain" (my own term, nothing medical about it!) because
of the severity. Could have been cramping in the ducts maybe, or maybe
yeast...we'll never know and I went on to breastfeed him til he was two
and a half years! Knowing what I know now, I'll always regret the
"mother encouraged" weaning.
Pearl Shifer, IBCLC