I'm curious what your on-call OB said? I am almost 35wks and all morning had the same pain you described. I never got a chance to call my dr this morning, and then after lunch it wasn't bothering me as much. I tried a heating pad after I first woke up, that helped a little. I also had nausea all morning, which also subsided by lunch time. I have been getting BH contractions all day, more than I normally get.
The weird part for me is that I just had an u/s and dr appt yesterday and while my dr does think I will deliver early, the baby has not yet dropped and my cervix is still closed. I guess that's why I didn't really think I could be going into labor one day later.
Have you felt sick along with your back pain? Pelvic pressure, or any indication your baby has already dropped? Do you get a lot of BH normally?