if it is not the right coverage or you can get seed and get flairup in other injection area that has been dormant if the antibiotic coverage is not correct. Goi to urgent care and either admit you injected B12 or play dumb. Amoxicilin is good for skin issues but the problem maybe that it is not staying in the area long enough. Thats why alot of docs will use I believe it is amoxicillian with potassium as it hangs around the infected area than just amoxicilin. Its an easy fix with the right antiobiotic, but if self treating didn't work it is time to see the doc.