I can only share my personal experience with you. A few years back I had a reaction to an antihypertensive. I developed hives and severe generalized itching. It was a full 7 weeks after stopping the medication before my symptoms finally subsided.

As for being on Benadryl and having allergy testing done, I was told not to take any antihistamines for three days prior to the testing. However, it was the skin-scratch testing that I had rather than blood work.
Why not give the doctor's office a call and ask whether the Benadryl will negate any results of your testing.

One can develop an allergy to just about anything at anytime, even to something that was never a problem for that person in the past.

Good luck!