Chalazion
The inflammation is caused by a sensitivity to a common bacteria found on your skin. When the openings of the small oil glands around your lashes become blocked by inflammation, a small tender swelling will occur in the lid. This is known as a chalazion.
The chalazion will often vary in size over a few weeks and can discharge spontaneously after hot compresses and lid cleaning. A small proportion of the chalazia will form hard lumps on the eye lid, which, if present for a few weeks can be incised and drained under local anaesthetic.
Sometimes the chalazion may cause a spreading infection along the surface of your lid, which might need a course of oral antibiotics, but this is rare.

How to prevent a chalazion
To prevent a chalazion from developing, daily lid cleaning is recommended.
A warm wet flannel with a few drops of baby shampoo should be used to clean your eyelids before going to sleep at night. An antibiotic ointment is often prescribed to rub into the base of your lashes at night once your lids are clean.
If your eye becomes increasingly red or painful after treatment or your sight becomes more blurred, you should see an eye doctor again or contact the hospital.