A26-year-old primigravida has brought her doula to the birthing center for support during her labor and birth. the doula has been her through the past 16 hours of labor. the laboring woman is now 6 cm dilated. she continues to report severe pain in her back with each contraction. the client finds it comforting when her doula uses the ball of her hand to put counterpressure on her lower back. what is the likely cause of the woman's back pain?