My goldfish's eye is swollen. Is it popeye or an injury? What to do?

It could be "pop eye". This can be caused by infection or trauma (aggression or from tank furniture).
Treatment will include medication and maintaining pristine water quality. Goldfish produce a larger than average bio load (wastes) which can quickly overwhelm a tank and filter. Ideally, one goldfish should have 20 gallons of water and 10 additional gallons should be added per extra fish. The filter should be rated for at least twice the volume of the tank. Check water chemistry using a home test kit or by submitting a water sample for testing. Most pet stores will do this for free or for a small fee. Aim for 0 ppm ammonia and nitrites with a nitrate level of 20 ppm or less. Partial water changes can be done to stabilize chemistry and maintain water quality. Use only aged or conditioned water that is temperature matched to prevent stress or shock. Goldfish are cold water fish and do best between 65F and 72F.
A broad spectrum antibiotic can be found in the Aquarium section of the local Petco. These include erythromycin, tetracycline or kanamycin. Brands include Kanaplex, Maracyn , FishMox or FishCillin. Follow label directions carefully and remove the carbon filter insert during treatment if indicated.