Aw, congrats sa pregnancy mo! That butt pain is likely sciatic nerve pain, super common sa third trimester kasi ng weight at pressure, a few of us moms experienced it din around that time. Try to stretch, switch positions frequently, at avoid sitting too long, one of my mum friends swears by pregnancy pillows between the legs when lying down, talagang nakatulong. But since week 31 ka na at persistent ang pain, better check with your OB para sure kung ano talaga ang cause at makakuha ng proper advice, kaya mo yan!