I work in infectious disease and wound care. It would be hard for me to tell what it is without examining it. Abscess, boil, cyst, etc. FYI, boils look like pimples, but abscess don’t.
The general recommendations for an abscess/boil is if it is less than 5mm (size of eraser head) then antibiotics alone should be able to resolve the abscess. Anything larger would require an I&D, incision and drainage. Cefdinir is a good choice. I prescribe it all the time. It has relatively few side effects compared to other antibiotics. Typically gut/digestion issues. Eat some yogurt or take probiotics, at least 2-3 hrs from taking the antibiotics. If it doesn’t improve, then you may need MRSA coverage (doxycycline, clindamycin, bactrim, etc.) or it isn’t an infectious process (e.g. abscess or boil).