Why did I have loss of vision and grey field in one eye briefly? Under stress then.

Many reasons. Its difficult to answer these kind of questions, not knowing some information. However, it could be migrains, tia, amaurosis fugax, visual changes induced by exercise, drop in blood pressure, etc. Stress in itself wouldn't cause a brief loss of vision, although it has been associated with central serous chorioretinopathy, which can cause a prolonged distortion of vision.