Mama, ang tightening and hardening ng tyan ay common talaga towards the end of pregnancy, pero since you're experiencing it almost the whole day AND may back pain pa, kailangan mo talaga ipakita sa OB mo asap, especially at 38 weeks, kailangan ma-check kung normal lang Braxton Hicks ito or something else na kailangan ma-monitor. Better safe than sorry na, alam mo na Sabi naman ng ilang mum friends ko, kapag persistent ganyan, mas okay mag-consult na lang immediately para peace of mind mo, the doctor will know what to do. Take care!