Hello and Welcome to Just Answer.
Thank you for the question. Answers given are for informational purposes only and are not meant to replace an exam by an in-person.
I am Dr. Sohaib, a Medical Internist (who deals with diseases of all the organ systems of body) and I will do my best to help you.
Most likely this is a result of un-noticed minimal trauma to to the eye lid, which can simply be in the form of a rub (during sleep) or pressure (from the pillow).
Another possibility is of an insect bite (mosquito etc).
If there are no other symptoms, you need not to worry;
Just watchfully monitor this over next few hours/day and see how this behaves.
Most likely this will disappear by itself, but in case if this increases in size, does not improve or gets worse (become associated with pain/swelling), have this examined by the doctor.
And do apply warm compresses over the affected area.
It will help.
I hope this clarifies and answers your question.
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