I have a hard mass along the inside of my leg. I've been on omnicef (cefdinir) for a possible abscess, didn't work. I'm lost.?

NEEDS I&D of abcess. A 'painful mass' is a sign of an abcess and the best treatment is 'incision & drainage' (I&D) of the infected material. Please get seen for this procedure. Antibiotics usually can not cure a large abcess, sorry. Hope you are better soon!
Diagnosis. You may need an MRI to have an accurate diagnosis. You may not have an abscess.