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Originally Posted by
lifteatsleep
Hey mate,
There are two possible causes here. First, is normal injection pain, this can last many days, especially if you injected too quickly or in a funny way. This is even more the case, if it is the first time hitting that muscle. So that is the most probable cause.
Second, it COULD be an infection. Is the area hot, red? (You already say it is swollen). If it is hot and red and this appears to be increasing it may be infected. Not to worry, anti-biotics will do the job in a few shorts day, but you must catch it pretty early before things escalate.
Good luck.