My BH for the last two pg have been what you are describing, really strong back pain and af cramping in my belly. With dd2 this did turn into pre term labour at 35 weeks, the difference was that they even stronger, were regular (5 minutes) apart and could be recorded by the monitor. I also had to really concentrate on my breathing, but most of the pain was in my back.

I hope it all settles down for you over the next few days.