What could cause recurring (abt 2x a month) of 3 - 10 red itchy swollen pimples on the right lower arm only and not anywhere else on the body? no contact with any unusual substances, no lifestyle changes, non-smoker, no drugs, no alcohol, no covid?

Your description makes me think about a condition called sporotrichosis (also termed rose gardener's or rose handler's disease) is an infection caused by the fungus Sporothrix schenckii, found throughout the world; it is usually associated with minor skin cuts and scrapes that occur when handling vegetation (moss, hay, wood, sharp-stemmed plants like rosebushes). It's treated with antifungal med.

Most typical is an allergic dermatitis. Could be something you leaned against or even a lotion someone was wearing when they touched your arm, or any number of unknowing exposures. Over the counter cortisone creams can help, as can covering the area with a bandage and neosporin.