According to WebMD, a bump under the eyelid is also referred to as a internal hordeolum and is caused by a bacterial infection of the eyelid's oil glands. In certain cases, internal hordeola drain on their own. If the bacteria cause a blockage of the oil glands, the bump may continue to swell. Blocked hordeola are referred to as chalazions and often resolve on their own within a few months.
The Mayo Clinic explains that hordeola and chalazions can be caused by inadequate hygiene practices that introduce bacteria into the eye as well as chronic conditions that cause continual eyelid irritation. Chronic eyelid inflammation is also referred to as blephartis and can be caused by skin conditions such as seborrheic dermatitis or rosacea.
According to MedlinePlus, additional symptoms of a chalazion include pain, swelling and warmth. Chalazions can often be drained at home with warm compress applications. Chalazions that continue to grow and do not dissipate on their own may need to be treated with steroids or surgically removed. Bacterial lumps inside the eyelids can often be prevented by scrubbing the eyelids each night with mild shampoo and applying prescription antibiotic cream to the eyelids. Complications of the condition include loss of eyelashes and scarring of the eyelids.