New Baby With Rash

My 1 month old daughter woke up this morning with a rash on the back of her neck,lower neck,upper back, and also a spot close to the middle backside of her head. Also when she woke up this morning she was very sweaty (no temp or signs of sickness).  Also her bedding has been recently cleaned with the proper detergents that are used for her clothes so I already ruled out detergents because the rash is only in these exposed areas. I think it may be heat rash because of her sweat, but Im not a doc. I included a pic, the big red spot on the right side of neck is a birthmark. I appreciate any help.