By history ,it seems to be an eyelash hair follicle infection/tarsal gland which has grown bigger and bigger with a collection of inflammation underneath. The anti histaminic Benadryl
use has done no good because it is infected .Even the bug or spider could have bitten and introduced infection.
You have not indicated whether you have had fever/ local temperature raised over the area. In case it is present then it confirms the presence of infection.
How about your age and whether you have family h/o Diabetes
. Kindly investigate and consult an Ophthalmologist who will drain out the collection of pus and prescribe an antibiotic as well.Pls do not neglect.
Wishing you a speedy recovery.
Dr Lt Cdr ASN Bhushan.