I suspect something in your tire.
I had a situation where my front tire went flat twice in two days. Now i know how to fix a flat and I ran my fingers along the inside of the tire after the first flat and found nothing, so i fixed the flat and off i went. After the second flat in as many days, again i felt the inside of the tire and found nothing. However my suspisions told me something was wrong since it wasn't a pinch flat and the hole was on the onside of the tube where it makes contact with the tire. This time i scrutinized the tire in the location of the hole by flexing it and sure enough i found a small piece of glass that was actually imbedded in the tire.
The moral of the story: Line up your tire label with your stem so that you can scrutinize your tire in the area of the leak.