This is an interesting question, but it is important to clarify that your particular problem does not appear to be an allergy at all. Allergies are reactions to substances that your body causes that are out of proportion to the danger, such as the very runny nose that comes with pollen, or the hives
and airway compromise that comes to some people with a penicillin allergy. They are generally marked by a release of histamine from mast cells, which is what causes the runny nose and hives.
The reactions often increase in intensity with each new exposure. Your description of your foot becoming hot, swollen, and itchy sounds more like an infection that required a trip to the emergency room for antibiotics. Whenever the skin is damaged, there is the potential for bacteria to enter and cause an infection, which it sounds like you had. The response to an infection is increasing blood flow, which causes heat and swelling. Itching can also accompany bacterial infections.
The fact that you took benadryl the 2nd time and had no reaction is probably just a coincidence, as you have likely been bitten by ants many times in the past without reaction. Please discuss with you primary care doctor
or allergist as needed.