Check your rim tape carefully. If any part of a nipple hole is exposed you will flat. It helps when mounting tires to line the valve stem up with the label, that way when you ffind the puncture you know where to inspect the tire. Flex it inward may show you the source of the puncture. I have had very small rock shards and radial wires cause flats before. They are hard to find and hard to remove. Keep looking until you find the cause because there is one. Tubes just do not go flat without a pinch or puncture.