HIstamine is a body response to an external insult OR an allergen. Benadryl is an ANTIhistamine, which means it counteracts histamine. The sooner you get it in your system the sooner it can work against the histamine and reduce the body's reaction to the HISTAMINE, not the insult.
Using that medical explanation, my belief is that the sooner you get it on the better. I am allergic to bee stings, and have to be very careful. If I get a sting and can get the benadryl onboard in 5 minutes or less, I have a less severe reaction. Best to avoid it all and really suit up.
Additionally some folks use liquid benadryl. Myself I take 100 mg as soon as I can, and put on topical benadryl and topical Vagisil (better anesthesia effect).
So the facts are out there now, between what the pharmacist said and this you should be able to make a decision.