It could be immune system. Search for Scleritis.
This has happened to me as well. It seems to be occuring in the left eye only and comes on randomly...there doesn't seem to be a trigger that initiates the swelling. I can feel major pressure building in the back of the eye and comes in the form of a major headache. It is very scary and painful when it falres up. Dr's do not seem to have answer as to wht it is and I'm getting frustrated and nervous about this...after all it is my eye and I dont want this to cause any damage to my eye. If anyone can help me out with what this could possibly be I would greatly appreciate it. Thank you.
I woke up about 3 a.m. one morning with a very painful left eye. I looked in the bathroom mirror and my entire scleria was swollen out of the eye socket causing dryness of the eye because my lid could not cover my corneas. I have glaucoma and am on 2 different drops a day and already have had surgery on my eyes. This has happened before and since but not quite as bad. My Dr. gave me an antibiotic ointment to put in my eyes to keep the cornea from getting infected. By the time I saw her it was the next morning and the swelling had gone down, but she could see creases in my scleria where it had been swollen. She did not know what caused it or why it happened. my vision is blurred when this happens and it is painful.
Call this morning and see if any Eye MDs (ophthalmologists) in your area have Saturday office hours, if so plead your case to come in for a medical evaluation. If they can't see you make an appointment for Monday, if you eye becomes painful between now and then go to the ER. If you wear contacts do not put them in.
Your description best fits an inflammed pingueculum which is a small bump caused by chronic sun exposure or contact lens wear.
JCH III MD