Need more information. Do you get a "pimple" at every injection site, or did it happen at just one site? Does this happen routinely? In other words, do you get a pimple or red bump at the site of every injection, be that a botox injection or even when you are giving blood? If it is a single event, then there is nothing really of concern, and it was probably just a transient event with spontaneous resolution. But if you answer yes to multiple sites of involvement and it occurs with most needle injections then we are talking about something very interesting.
Pathergy is a reactionary process that often manifests as a pustule that can progress to frank ulceration. It is most commonly associated with Behcets, pyoderma gangrenosum, Sweets syndrome, and inflammatory bowel disease. If this is the case, you should see a dermatologist or internal medicine physician to have this evaluated further. I hope this helps!