blistered and scabbed ulceration on arm

[boots]

I have this blistered and scabbed up thing on my arm its about 1 inch X 1.25 inch and slowly getting worse. I have antibiotics for it but it ain't doing anything
Maybe an infected spider bite but maybe something else and its itchy.

[boots]

I have started putting Epsom salts on it to draw the fluid out of it