Im an rn and did major studies in midwives and doula training. Most women experience spotting at the beginning of their pregnancy. Believe me im pregnant with my third and am just as parinoed everytime I go to the bathroom. We can all try and easy u because its probably fine but the only one that can easy ur mind and nerves is u. Itll make u feel better if u go to the doctors but most likley if there was just spotting that one time and the back ache is gone then u should have nothing to worry about. Relax and hope u feel better.
I had some spotting in the beginning of my pregnancy, im 12wks 5days. Be cautious of cramping and an increasing amount of bleeding. You can always go to the dr for blood work, they can test your hormone levels over time. Take it easy and do your best to relax. Good luck.