Where on the tube is the flat? Is it occuring on the outside of the tube, or on the inside where it meets the body of the rim? Is it happening near the valve stem? Does it look like a long slit or is it a pinhole? This information can help you narrow down how the flat occured.

I'm 245 lbs., and prior to building up my most recent commuter I was running on 23mm tires. Never got a flat because my tire pressure was always kept over 100psi and I purchased a puncture resistant tire. I moved up to larger tires because they're much more comfortable, but the original tubes are still going strong, nearly a year later.