She may have a food allergy. It could be from something she has been eating, some object around your home she has sucked on or maybe someone ate something she may be allergic to and kissed her. I have had food allergies all of my life and when I was a baby my grandmother would eat seafood and I'd get a rash when she would kiss me goodbye. As a child I was even allergic to certain preservatives in many foods. I either grew out of them or preservatives changed. Either way I'm out of that one, Thank God ! My daughter (7 months) began eating baby food and I noticed after giving her aa gerber turkey dinner last week her chin broke broke out in a red rash. It did go away after about three days. Anyway, Check food and skin allergies. With skin allergies something could simply brush against her skin and cause a rash. Growing up I'd always get nailed by the perfume women in department stores. They'd spray me and next thing I knew I'd have sinus problems, wattery eyes and a rash. Allergies can come in many forms and from many things.
Good luck, J.