your BB is very light, my BB is 6.43kg when she's 3 mths 2 weeks old.
My BB had rash when she's new born. Doctor gave him aqeous cream (where you can buy in Mannings too, $30 a big bottle), but didn't work well. Later uses Cetaphil (you can also buy in Mannings too) which is good.
do you use warm water to wash your bb's face? remember to use cold water no matter how cool the weather is. put on cream immediately after you have her face washed, and perhaps a few more times when the humidity is low. on bathing, don't use too hot water. also put on cream on her body after bathing.