I dont think it was anaphylactic shock , as it would not be that immediate with the injection given intramuscular . I suspect the penicillin injection was given in the Interclavicular air sac or the anterior thoracic air sacs- located in the area you described. Also because chickens have air sacs found all over the body it is very possible that it was suffocated during restraint... i know it sounds impossible but it happens all the time. it does not take any great amount of pressure so it could easily be done on accident.
You should not feel guilty, as your intentions were good but it should be reflected on as a lesson learned - if you do not know the
anatomy ,chemistry and physiology of poultry you should not be performing at home medical care . Unfortunately the feed stores do a lot of encouraging and obviously supplying, but the best advice they could give someone is to seek professional help