Try to keep track of whenever you feel your back hurting and cramping pain, and the intensity do they always feel the same or continue to intensify
Sounds like your having contractions you should call your doctor and discuss whats been going on
I am 38 weeks and three days my lower back is always hurting too and the pelvic pressure sometimes will stop me in my tracks for a second I've been having stomache tightening for the last three days and mucus plug coming out for the past 4 days as well and sciatica nerve pain in my right leg I think it's just almost time lol also last Friday I was 4cm and 50 percent effaced