it sounds like u experienced a temporary effect of increased intrathoracic pressure when straining, called a valsalva effect. this in turn causes increased intravenous pressure in your head, which includes the eyes, and this theoretically could cause a temporary lack of blood flow to your eyes and dim the vision, though i have never seen this. and if it is true then i urge u to have your eyes examined to rule out any signs of abnormal vasculature, or blood supply. but i am sure that u will be fine.
key words: valsalva