Uy, mama, I'd really encourage you to call your OB or go to the hospital right now para ma-check properly, back pain + tight belly this close to your due date needs a doctor's eyes on it, hindi lang online advice!
A few of us moms had similar things and it turned out to be different things, some were just Braxton-Hicks, iba naman may concerns na kailangan ma-address agad. Better safe than sorry, especially ngayon na malapit na ang December 30 mo. Tawagan lang agad ang clinic mo or go to ER kung wala kang maipon-contact, they know you and baby best! You've got this, mama!