Looking at your photo: there's a tight cluster of roughly 15-18 small round lesions in one localized patch on the inner/medial upper arm, sitting on a faintly pink-orange (mildly inflamed) base. Some of the bumps look pearly/light in the center, others have a dark red-brown or black crusted center, and there's one separate paired lesion with dark puncta just above the main cluster.

That pattern - a tightly grouped cluster of papules/vesicles with central crusting on an erythematous base, confined to one small area - has a few likely explanations:

Most likely: multiple insect bites (papular urticaria) Fleas, bed bugs, ants, or mosquitoes often bite repeatedly in one small area over a single exposure (e.g., resting against furniture, grass, or bedding). This produces a cluster of round papules, each with a tiny central punctum (the bite mark), which can crust over as they heal or if scratched. The tight grouping in one confined patch, with lesions of varying stages of healing (some fresher/paler, some darker/crusted), fits this well.

When to see a doctor promptly: if it's spreading, very painful, blistering with clear fluid rather than just crusting, associated with fever, or not improving/worsening after a few days - especially to rule out shingles or a bacterial skin infection, both of which are treated differently.

I can't make a definitive diagnosis from a photo alone - if this doesn't resolve on its own in a week or two, or if you're worried, it's worth having a clinician or dermatologist look at it in person, since some of these (zoster especially) benefit from early treatment.