Major discomfort is very common when you first start using gear or have taken time off of it for a long time. Oil based depots can be really painful especially of you. Short acting esters can also cause a lot of pain. My first shots felt like i was hit with a bat! The muscle will get more used to the shots down the road. All that said, if the site is turning bright red or another color. If you are getting this and have a mild fever plus feel sick, you may be developing an abscess. This is possible even with the cleanest of gear. I would have to see it and probe further in order to figure that out. Advil and ice will help if this is not the case and be sure that you rotate sites. Don't use the same site until after the pain and knot has subsided, no more than 1x wk per site.