I too have a PCN allergy and had never been prescribed Keflex until once about 5 years ago, when I was told that there would be no problem. After the second dose I began to experience a precursor to anaphylaxis where my lips and tongue became alarmingly swollen, and I began to have difficulty breathing.
I ofcourse immediately discontinued the medication and required benadryl po for over a week before the swelling completely receded.
This was during a period of time when I found that a lot of providers were questioning claims of allergy. During that 2 year period I don't know how many times that I heard " So how do you know that you are allergic to PCN?" It seems that I heard it every time I saw a MD.