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If the bruising has occured for the first time and is resolving it does not seem to be a cause for concern. However, if it recurs then I would advise you to see an ophthalmologist for an examination. It could have been to an insect bite or some infection which is resolving. Thanks.
It could have been a sub-conjunctival hemorrhage which usually requires no treatment and resolves on it's own. But if it is recurrent then you should see an ophthalmologist. Thanks.