There can be many reasons for that, but it seems that until now you did not get an infection, so the problem is not major. Perhaps it is the alcohol that did not get completely dry before injecting, or may be you are injecting too fast, or in the wrong location....Try changing these things one by one, and see if there are improvements. I would not be all that worried, as you don't seem to have an infection. However, it is best to deal with this issue.