Discussions taking place within this forum are intended for the purpose of assisting you in discussing options with your vet. Any other use of advice given here is done so at your risk, is solely your responsibility and not that of this forum or its owner. Before posting it is your responsibility you abide by this Statement
Often it is an eye scratch or poke, but it can have other causes. You need to see a vet for a diagnosis and proper treatment. The injuriy is not necessarily visible to the naked eye and may likely need some dye and ultraviolet lighting at the vets.
Thankfully, most eye injuries come back down pretty quickly again, if not quite as quickly as they have come up.