My right nostril is swollen inside. Is it a pimple or an allergic reaction?

Nostril bump. Generally, pimples inside the nostrils are painful/tender, while allergic reactions are itchy. Most likely, you have a pimple or an inflamed (+/- infected) hair follicle. If your condition worsens, you develop additional symptoms, or it persists unchanged for 2+ weeks, you should see your doctor. If you develop swelling elsewhere, difficulty breathing or swallowing, ... Go to the er!
Is it painful? Pimples are painful to touch, pushing on your nose would hurt. Allergic reactions that cause swelling in the nose are not painful.