Aww mama, 31 weeks na! Yan ang Braxton-Hicks contractions kaya normal lang, lalo na kung hindi talaga sobrang sakit, nakaranas na din kami ng ganyan ilang beses a day, minsan buong oras pa. But with the back pain at parang naiipit na feeling, I think mas okay kung i-check mo na talaga with your OB, kasi sometimes it's just ligament stretching pero better safe than sorry, especially since you're having trouble walking na rin. One of my mum friends had something similar and turned out okay, pero seryoso yung OB-check para ma-rule out any nerve pinching or positioning issues. In the meantime, try warm compress sa back at magrest as much as you can, pero don't wait, book that appointment na mamsh!