Unexplained swelling of the eyelid without any predisposing disease is frequently caused by allergies. An allergic reaction can occur suddenly, even in a person with no previous history of allergy. This swelling is called angioedema. Diagnosis is by clinical examination and complete blood tests, electrolytes and renal function tests. Treatment is by antihistaminics and steroids.
Other possibilities of such swelling may be acromegaly, Crohn’s disease, herpes and myxedema. It is very difficult to make a diagnosis without examination. So please get yourself examined by a physician/allergist and the diagnosis confirmed.
I hope that helps. Take care and please do keep me posted on how you are doing. Warm regards.