Here is a repost, but worth it. The key is time and the 72 hours mentioned is pretty accurate.
Infection and abscess.
So now to the most serious reason for injection pain. An infection will start in the same manner as tissue irritation with local pain and swelling, with heat and redness around the muscle. The major difference is that after 72 hours tissue irritation should start to subside, if the area is indeed infected this pain and swelling will get worse. The swelling will change in nature becoming more systematic and edema will start to form under the skin becoming softer and more spongy (as described with a lymphatic puncture).