It is possible to develop allergies to things you have used before.  In fact, to have an allergy, your body has developed a sensitivity to that "thing".  Often times that comes from a previous exposure.  I just had an anaphlylactic reaction to Avelox (antibiotic) that I have used MANY times in the past - even was on it for a month about 4 yrs ago for serious sinus infection.

One way to test, draw a circle on her arm when you put on the emla.  Check it over next 24 hrs to see if that is where the rash appears.......

Good luck!!! I know what it is like to have "weird" allergies - I am hyper sensitive to a LOT of things!