There is an app called "Should I Sleep" in the App Store that watches for movement and will not let the computer sleep or will wake it if there is movement (or sound or other indicators) but you would know if you had that installed.
I have encountered similar symptoms and like Grelber it always turned out to be an overly sensitive keyboard, mouse, or trackpad that I was inadvertently jostled it or the desk it was on when I stood up. I suppose it could also be caused by a loose or otherwise faulty USB connection that was joggled when I moved. Try unplugging your USB devices, including hubs, and turning off any BlueTooth or other RF devices one at a time and then stand up just as you normally do. Not only will you get a good cardio exercise from this, you will likely discover the errant device or connection.
