Ay common yan sa pregnancy, lalo na kapag tumatagal, marami sa amin na bumubuntis nag-experience ng ganyang back pain, especially sa lower back at butt area kasi lumalaki na ang baby at nagbabago ang posture mo! But since it's nagging and you're worried, I'd really suggest checking with your OB para ma-rule out kung constipation lang or something else, kasi better safe than sorry sa first trimester. In the meantime, try warm compress, proper posture, at avoid standing too long, a few of my mum friends swear na nakatulong talaga yun when they were preggy.