Whilst it could happen later down the line, it does not sound like this is due to an infection. Even the most virulent, pestilential contagion, whilst some can kill in hours (such as pneumonic plague, or the now extinct sweating sickness for example) there is no virus nor bacteria on earth that can be introduced into the body, via a wound etc. and cause symptoms within minutes of exposure.
That sounds like a direct physical reaction of some sort to the injection (and its contents, I mean), a blister arising within minutes isn't going to be an abscess. Not when someone shoots up and then 100s of seconds later it has already raised a blister, no way that is due to infection itself, although slow wound healing could be due to infection from an open wound.
Are you making certain to register the shot into the vein and that there is no missing? does it happen even when you sucessfully register, and stay in the vein throughout the entire shot?