Ay alam ko yan, super uncomfortable talaga! Ang back pain at frequent Braxton Hicks contractions (yung tightening ng tyan) ay pretty common sa third trimester kasi lumalaki na talaga ang baby at nagbabago ang posture mo, pero since it's happening a lot in one day, better check with your OB just to be safe na walang complications, hindi naman dapat maging worried pero mas peace of mind lang if doctor confirms lahat okay. Try warm compress sa back, good posture, and rest when you can, pero let your doctor know agad para ma-monitor properly. Sending you comfort, mama!