Heat rash or worse?

It started in her hair line, I asked my mom about it and she said not to worry babies skin are sensitive, start to worry if she acts weird or has a fever. Then it moved to her shoulders in 2 days. She isn't acting weird and doesn't have a fever, she does rub it on whatever she's laying on like it itches. If it is a heat rash I know that you don't want them to over heat and to let her air dry, but is there anything else I can do? I feel helpless. It sucks!....