The most common reasons for the problem you described are an infection or foreign object in the eye.
An infection is normaslly indicated by a pulling sensation in the eye, whereas an injury will present as painful as will an object in the eye.
A gp should be able to determine what the most likely cause of the problem is, but unfortunately a split lamp examination remains the best way to diagnose eye problems.
Carefully rolling up her top eye lid may reveal any items in the eye.
Flushing the eye with saline may help. Keeping the eye covered may also be worthwhile.
Mike