What could a single red blister on my left arm that does not hurt be?

A single red blister that does not hurt on your left arm could be a number of things, including:

- Insect bite or sting

- Hives

- Contact dermatitis

- Atopic dermatitis

- Psoriasis

- Erythema multiforme

- Drug reaction

- Viral infection such as chickenpox or shingles