Not an infection. Some anabolics like T propionate, or primobolan, or even sustanon(has prop in the mix) can cause reactions like this. Some people take advantage of the swelling for site enhancement, injecting into smaller less developed muscles(equal amounts either side of the body. High concentration/ml can also cause swelling. Could also be the deposit of steroid not dispersing, more common for long estered testosterone.
Nothing to worry about unless its making you feel sick, or is effecting your mobility. Usually goes away in 4-7days, but sometimes as long as 2 weeks, gradually reducing in size over time.
Ethyl oleate used in some steroids as a solvent or carrier oil can cause massive painfull swelling(though not an infection) if it doesn't react well with you.