I have a 2-inch red patch on my upper arm, filled with small bumps/spots/pimples. these bumps are translucent and have a black dot. they are painful. ?

Shingles? Herpes?: If it appeared and developed the pimple-like bumps over just a few days, it could be herpes zoster (shingles). Less likely is herpes simplex virus (HSV). It's an unusual location for HSV, unless perhaps exposed to infected persons in contact sports: "herpes gladiatorum" occurs in wrestlers, rugby players, etc. You should see a doctor right away, both for accurate diagnosis and maybe antiviral Rx.