A dentist told me that he has given xylocaine- adrenaline infiltration for some dental procedures to a woman on two different visits.The patient was alright in clinic on these two days but developed skin rashes after 4 to 5 hrs  which disappeared of their own in next 10 to 12 hrs.Is it an allergic reaction? If so, why delayed appearance of rashes?

What should be line of action in the third visit.?