A foreign body sensation in the eye may be due to an actual particle in the eye which cannot always be easily visualised by the patient in a mirror especially if the foreign body is located under the upper lid and needs to be seen under slit lamp evaluation by your ophthalmologist. Another cause for a similar sensation in the eye is a corneal or conjunctival abrasion which can also manifest as a pricky, scratchy sensation in the eye. The history you give of it having begun when you went to scratch your eye could indicate an injury with a fingernail. It may also be an eyelash dislodged and on the cornea but that often washes away with the excessive tear formation due to it. You require a slit lamp evaluation by an ophthalmologist to look for the presence of a foreign body and following that a fluorescein staining of the cornea to check for any corneal abrasion.