not sure if it's an allergy

My DD has a MSP allergy but lately with the baby food she has been getting a dark red spot on her temple. I read all the labels and nothing contains milk or soy and we do not use Gerber because they may contain soy.  It goes away in about an hour and doesn't seem to bother her. I have noticed that it does not occur when I give her homemade baby food. Could she be allergic to the preservatives?