D7100 buffer too small
And make sure that you're using at least a 45MB/sec card, 95MB/sec if you want the absolute maximum buffer throughput for continuous bursts. Search in this forum for a recent thread on buffer size vs. card speed.
You should always buy the biggest, fastest cards you can afford. Personally I would never buy anything below 95MB/sec for a camera like the D7100 or any body that was UHS-1 compliant.