A 26-year-old primigravid woman at 42 weeks' gestation comes to the labor and delivery ward for induction of labor. The prenatal course was significant for a positive group B Streptococcus culture performed at 35 weeks. Antenatal testing over the past 2 weeks has been unremarkable. The patient is staed on lactated Ringer's IV solution. Sterile vaginal examination shows that the patient's cervix is long, thick, and closeD. Prostaglandin (PGE2) gel is placed into the vagina, and electronic fetal hea rate monitoring is continueD. In approximately 60 minutes, the fetal hea rate falls to the 90s, as the tocodynamometer shows the uterus to be contracting every 1 minute with essentially no rest in between contractions. Which of the following was most likely the cause of the uterine hyper stimulation?