I have another thought.... This reaction may not be due to histamine, but bradykinin instead.

Bradykinin is another biogenic amine that is released by various triggers. Usually physical ones.

It can build up in the body in people using ACE inhibitors for blood pressure. So Kev...if you are using a drug like this, you need to talk to your doctor to get another family of medications for any blood pressure treatments.

Some people have a hereditary genetic error in the metabolism of bradykinin. (This was discovered for me, when I had a huge reaction to Lisinopril for blood pressure). You can swell, turn red, and hurt tremendously. This is tested for with blood tests for Complement #4 and C-inhibitor.

The blow to your leg, is enough to release the bradykinin from cells and the red swollen appearance is typical. People with this problem also react severely to stings and bug bites.

Many doctors are unaware of hereditary angioedema...and so many people go without a diagnosis. This type of angioedema does not respond to antihistamines. That would be a clue that yours might be due to bradykinin.

Here are some links:
http://www.haea.org/patients/what-is-hae/

You can see from some of the photos, that the swelling can look very like some RSD...

I have had my left hand swell up for weeks, following an IV for a test.
Also I typically get mine in the abdomen and that is called GI angioedema. I was hospitalized once for 10 days with an attack in the 1970's....back then all the doctors were stumped, and I narrowly avoided surgery then. (they found twisted intestines though, saying I had those from birth). Sometimes I get it in my throat, but I have never totally closed up there yet.(thank goodness).