#1 Your "nocireceptors" (pain receptors) are located
deep in that particular area, and may branch up to
the surface. This is just your genetics - you have to
live with. If you frequently inject in that area you're
going to be stuck with lots of scar tissue aswell.
#2 Possibly there was some bacteria on syringe (it happens) If you wipe the area twice with the same
swab, it basically puts some strains right back on.
Pseudomonas is particularly resistant to alcohol. Also,
the solution itself may not be sterile (i.e: IP's).
Basically, if it is an infection, monitor it closely - look
at the diameter of "red" around the site, and see if
it is warm to the touch. Currently you are probably
suffering from an innate inflammatory response. Macrophages, neutrophils, and the complement system
are trying to destroy whatever breached your skin.
If the circle gets bigger than 3cm in diameter go to
the doc and be honest. I actually waited until my WHOLE leg was throbbing red, and was running a serious fever. The point is just be careful. You
will definitely have to rotate your injection sites (maybe to the delt, other glute, or quad)