Lots of things to consider. Were you standing with your knees locked (and quads tight) for an extended period of time? Squatting? Sitting? Did you pass out right when you stood up?
Meds, climate, etc all can contribute. Glad you got checked out but that sort of thing can just happen from time to time as well if you're not aware of why. There doesn't have to e nothing medically wrong with you for that to happen. Numerous things can cause the blood to leave your upper body quickly enough to pass out even if you're extremely fit.