Pregnancy can get very uncomfortable in the later stages, and some moms-to-be suffer more than others, depending on the position their babies are in. Women who are pregnant with subsequent babies also tend to report worse symptoms because the ligaments in the back and groin may have been stretched out during previous pregnancies, which means there's less support for all the pressure your baby's putting on your body.
To feel better, try placing a heating pad on your back for 10 to 15 minutes every hour or so (don't leave it on for more than 20 minutes at a time, though, since the heat can raise your baby's temperature too high). You can take acetaminophen (like Tylenol) to dull the discomfort, but steer clear of medications like ibuprofen and aspirin (like Motrin or Bayer), which you should always avoid during pregnancy. If the pain is making you toss and turn at night, you can also consider taking an antihistamine like Benadryl at bedtime to help you sleep better, if your doctor approves.
Try to remember that this pain is just temporary and you'll feel much better in only a few weeks, when you'll also have a brand-new baby to welcome into your family!