How do I treat a pimple under the eyelid.? Have a small pimple under my left eye that doesn't want to go away. The pimple feels like a hump.

Chalazion. A persistent bump under the skin of the eyelid is usually a chalazion, a cyst which encloses the debris from the battle of your body to overcome what starts as a stye. If small it can be ignored but if it embarrasses or bothers you, see your ophthalmologist who can remove it in a quick office procedure.
It's probably. A chalazion. Use a warm wash cloth frequently (at least four times per day). If this does not make it go away in two weeks, then see an ophthalmologist.