Try not to worry hun. Its still early, the monitor could not be sensitive enough to hear it yet or baby might be in a different spot than what you think or could be 'hiding'. Wirh DD my MIL bought one at8 weeks for me and OH was mpre excited about it than I was so we(well he) tried it. We tried everyday some days we heard something other days we didnt, even after just coming back from the docs knowing everything was fine.
As for the back pain, I wouldnt worry too much. Although babys still so small, just like having to pee all the time, it still can cause back pain. At least that's what I was told with DS.
if it helps any, Im right thrre with you on the back pain. And have cramps as well, not as frequently as at first but oh so stabby like. Lol.