The following contains graphic material. This is what can happen when you don't follow proper injection procedures or you buy gear from a non-reliable source. Either way, be smart. Read up on injection procedures, use proper injection technique and make sure that you obtain gear that isn't garbage in a vial. The following pictures show how a person injected gear into his tricep muscle, and how it went horribly wrong.
Picture 1
This is an infected injection site. It is red, raised, and i'm pretty sure the guy would say it's painful. Your body is reacting to the injected material and is causing all the symptoms.
Picture 2
Here's a side view. Notice how raised the injection site is. Your body actually surrounds the material that is injected, causing the area to become a pocket that contains the gear injected. This "pocket" is termed an abscess.
Here's a depiction of of an abscess.
Picture 3
Since the injection abscess is so deep in the muscle tissue, there really isn't anything you can do with this except go to the ER where they will cut it out. This is the only way to get rid of the abscess.
Picture 4 & 5
This picture shows how wide and deep they have to cut to remove the abscess.
After the abscess is removed, you'll be given antibiotics to reduce the CHANCE of infection. Now, you'll have to wait a pretty long time for that site to heal.
Please, for your own safety, use proper injection procedures and make sure you are getting your gear from a reliable source. Your health and safety is important.