Hi.. I can understand your concern. But let me assure you its normal and will go away. Since, it is summer now, daily bath your baby with proper baby wash, try putting over cotton clothes which will be comfortable, airy and will absorb sweat. Do not put any powder or oil. You can use mild moisturiser. It will go away in few days.
Health Tips
If you face any difficulty even with these tips, you can have a review with your Paediatrician.