Preterm labor symptoms

Anyone who has gone into preterm labor, what were your symptoms? Is there usually bleeding/obvious contractions? Last night my lower back started bothering me along with period like cramps. Both of which I know can be totally normal. It was enough that it was hard to get to sleep but not bad enough to wake me once asleep. It was been happening again since 2pm, it’s now 6:10pm here. I have also lost bits and pieces of my plug the last 3 days. No blood at all, but I’m confident it was my plug as this is my 3rd. I lost it on and off for 10 plus weeks with my second and still went 6 days late. The only risk factors I have is my last pregnancy was a little less than a year before this one. Also doctor mentioned my cervix length was only around 3cms at 10 weeks when she did a pelvic exam. She was not concerned though. I will be 20 weeks tomorrow. My nap didn’t stop the pain so now I’m taken a warm bath to see if that helps. Wondering if it most likely just normal pregnancy pains or if I should go sit at the ER.