Calamine lotion is quite commonly used here for baby heat rash, a few mums I know swear by the Guardian calamine lotion specifically for neck and shoulder rashes, saying it helps reduce the itchiness so baby stops scratching and making it worse. Some also combine it with a little aloe vera, either in a lotion form or applied separately, which seems to help soothe the skin further, though a small number found that if the rash had already progressed beyond just redness, the calamine alone wasn't quite enough on its own. The key thing most agree on is keeping the neck area as cool and dry as possible, loose clothing, good airflow, and patting dry after feeds, so the rash doesn't worsen while you're treating it. That said, since it's already changed in appearance from the initial heat rash, it's really worth getting your PD to take a look, just to rule out anything else that might need a different treatment.