This is likely due to instability while injecting. If your hand moved around more than necessary, it's likely the tip of the needle (which is a blade) caused more damage than needed for an injection. Tissue does not like that and reacts. This is common when you first begin injecting until you have more experience. It will probably heal or come down in 3 days or so. If the site continues to worsen after 5 days, I recommend that you seek a physician's attention to rule out an infection.
I'm sure you may have heard of the "Virgin Muscle" theory, rest assured, there is no such thing.