I had the same in one of my younger adults. I treated them a week before Potomac, and she wound up with what looked like a burn. I'm hesitant to use the regular Frontline, so I opted to use Ultra Shield Equine spray. So far, it has worked well as a repellent and no skin irritation.
You can't always trust when you buy it on line. Many manufactures won't back the product unless its sold by a vet. I always buy it from my vet. May cost a little more but it gives me some piece of mind.
For whatever reason site reactions seem to occur more frequently with Plus than Regular. Also dogs that react to Plus often times will not with Regular. Just what we heard "in the trenches" calling on clinics. Some clinics switched completely back to regular because they were unhappy with the increase in incidence of site reactions. It is a application site reaction to the carrier agent in the product. Hope this helps~!