A subconjunctival hemorrhage is quite frightening, but when following vitrectomy, it is quite benign.
Similar to a Bruise
A bruise of your skin and a subconjunctival hemorrhage are the same thing. In both cases, blood is trapped beneath tissue. The difference is that with the eye hemorrhage, the blood is trapped underneath very thin transparent tissue.
Hence, the bright red appearance. There is no muscle or skin to mask the bright red color.