Red spot on my 7 week old's neck?

My daughter has a red spot on her neck, under the folds. It appeared earlier today but it was pink, and there was some milk residue in it. I cleaned her whole neck and didn't think too much of it. About 30 minutes ago, my fiance noticed it and it was much redder. I tried drying it out but she is really sensitive when we touch it. He thinks it is just heat rash. I think we should call the doctor but I tend to be a bit neurotic when it comes to things like these! Am I overreacting? Does anybody know what it is and what we can do to help soothe her?