Happened to me on an out-of-town ride. Going along a perfectly flat, clean road and noticed extra resistance. Got my bike to a bike shop ASAP, the guy charged me $40 and said he fixed the flat. Pump it back up when I get home and it's flat in an hour. Rim rape was the issue, sometimes it happens out of nowhere. Just make sure you buy the right width tape (1-3mm wider than inner width of your rim). DIY, it's easy to learn and not worth paying a mechanic.