Sometimes if you don't do the shot right, some of the T can encapsulate. However, this is a little late in the piece after your shot for this to be happening (the body usually breaks it down within a couple of weeks).
How do you fill your needle? Is it pre-filled or do you fill it from a glass ampoule? If you snap the ampoule open, sometimes a tiny shard of glass can get up into the syringe with the fluid and will then get injected with the T. The glass shard will normally work its way out though. If this is how you do your shots, then this is a possibility.
The other possibility is that it's a co-incidence. Maybe you got an ingrown hair at the site, or a mosquito bite. It seems unlikely that it would take 18 days to manifest.