Prickly Heat rash near bicep?

I have a prickly heat rash near bicep, I think. My doctor said it is probably heat rash and it breaks out whenever I sweat or am in the sun for to long. I know people say don't sweat or go in the sun but I do sports and I sweat and its summer. How can I get rid of it I am getting annoyed with it because I have had it for so long. I also have molluscum if that has anything to do with this.