Sounds like it could be eczema or heat rash.
If it spreads, or the symptoms get worse, see a doctor.
I think you need to consult with a doctor as soon as possible.
I think it is not a big issue, just apply some simple cream on it and it will go in one or two days. If you are thinking that it should go in one day then I am afraid it will not go. It will take its time even if you will not apply any cream to it then also it will disappear in 3-4 days.