Just to verify, you did check the pressure right before the ride correct? I can pump up my road tires and they will be down 20-30psi in a day or two. The air leaks out of road tubes pretty quickly. Other than that, I agree with others to check your tires for something that is still in there from a previous flat. Also, why not carry a small flat kit with you out on rides. That will prevent being stranded when you flat.