This is exactly what happens with me.  It started about 25 years ago and as I am realizing now it is still happening.  When I "feel the burn" with anaerobic weight lifting I will experience swelling of the muscles I over exerted about two days after the exercise typically along with diarrhea then the next day I will get hives all over except the muscle group that swelled.  The hives will usually last for a couple of days.  Once my blood pressure dropped so low that I passed out. 
I've been good about keeping my exertion level under the threshold to trigger this but the other day I started a strength and conditioning class and too many of the exercised focused on the triceps.  Now my right upper arm is painfully swelled and I am just waiting for the hives to start.
I do have a twist though.  About the time this started I was diagnosed with Eosinophilic Gastroenteritis which is the presence of a type of white blood cell in the GI tract.  Eosinophils are associated with allergies.  The best explanation I ever got was from an allergist who suggested that I may also have eosinophils in some of my muscle groups which somehow get triggered when I tax that muscle.
Just like the rest of you I know that it has nothing to do with something I ate or something in the environment, this came from the exercise.  If anyone has a theory or suggestion I would love to hear it.