What was being injected? Have you seen a doctor?
If there it was bruised and there is a knot it probably is a hematoma. Sometimes these can take a few months to go away. If it is a hematoma the doctor can remove the knot of blood that gets left by using a syringe and needle and aspirating it out. This will make it heal quicker. However, it should go away on its own. If it isn't improving over time, then I would see a doctor
There wasn't an immediate bruise. In fact, there wasn't anything more than a small sore spot where the needle entered. The spot only became more noticeable in the last week or so. Especially when I outstretch my arm. I am planning on seeing a physician asap to see what it is. The solution being injected was a generic version of morphine I believe. A couple sources pointed at the possibility of an abscess but it doesn't bulge out or anything.
If it is an abscess you could a fever but not always, also it would be red or have pus and swelling can be an indication. But if it is deep inside it maybe a pocket that is infected. It is hard to know that. It could be that the muscle was bruised. Is it more noticeable in that it is getting more red or more painful or what do you see? It is a good idea to double check it with a doctor. If it is nothing and just internal bruising that will go away at least you will not worry about it.