Bulldogs World

We have a 4/12 month old bulldog who has  recently developed hives.   He is not in pain, or uncomfortable however he does have bouts of itchiness . To play it safe we took him to the vet to have him checked out and it appears to be what we were expecting -  an allergic reaction to something.  Problem is  we are not sure what he is reacting to.  We have pretty much ruled out his food and contact allergies, as there have been no changes from before he got the hives.  Then talking to another dog owner she mentioned having a very similar problem with her dog and after months of  tests etc. she took her dog to a groomer who asked what type flea control the dog was on (program) and told her that it was most likely the problem. Because she had tried everything the vet had recommended with no improvement she stopped giving the dog program and within weeks the dog returned to being happy and healthy.  After she mentioned that, we realized that it was about 2 days before we noticed the hives that he was given his first monthly dose of Sentinel (oral flavor tab).  Has anyone else had any issues with Sentinel?  If so how long did it take to clear up, ie did take a month or more for the medication to leave his system.  

The hives don't appear to be getting better or worse (they have moved around a bit but the severity has not changed).  He is being given benadryl and it does appear to help reduce the hives temporarily.