The cervix can hang low for various reasons, from the more serious like uterine prolapse (much less likely if you haven't had a lot of children) to it simply being your ovulatory time of the month. You might google "cervix low" and see what you find.
I too deal with painful sex depending on different days or positions. My doctor and my mothers doctors says its related to where the cervix is located. If it is indeed hanging low..he suggested kiegal excersizes to strengthen you vaginal muscles. My mother actually ended up having surgery due to what AnnieBrooke said, Uterine Prolapse (which is less likely if you've never had kids) and she was also right about your ovulation time because the womans cevix changes in size and such during this period of time.