Nees advice on neck rash?!

So my baby is 5 weeks old and I noticed he was getting a smelly neck and found milk in the creases so would wipe it away and bath him in just wanted to wash it all away and then I tried putting cream on just to sooth the area but found its just getting worse it's all bumpy and spread and even when I have made sure no milk has dropped dripped down there when he wakes from a nap for next feed it smells so foul again when I pick him up! It's like a cheesy infection type smell so I'm starting to wonder perhaps it's not milk that's the issue? Has anyone experienced this? I just feel like it always smells even if I bath it and dry and put cream on or don't put cream on we have our six week check coming up so will ask then too but any advice would help. X