OP you're putting a compound into your muscles. If you're new or newer to AAS than this is going to cause discomfort. I've cycled AAS for 12 years and I still get minor PIP with certain compounds.
Just as Raptor suggested... Make sure to follow good pinning protocol and not move the needle around. Obviously that advice is standard operating procedure but pay close attention when you're pinning because this might be causing the source of your pain.
Its not uncommon to feel discomfort for 48 - 72 hours after pinning. If it gets warm to the touch or swells a lot than you most likely have an infection. Infections happen every once in awhile even when sterilizing everything. You should watch the infection closely but you don't have to run off to the ER right away. But if the infection lasts for more than a week and is getting worse, ie... Running a high fever, sweats, chills, etc... Then you should get checked out. I've known a guy that's gotten merca twice from AAS. Point is... Just each it carefully. Also, if you are getting the infection with every injection you could be running dirty gear. I'm not sure who you're running..
good luck. Remember brother... Pain is part of the game