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A: It certainly is an unusual presentation. If you were a question on my OB final exam, I suppose I'd answer it something like this: The different possibilities would include: 1. muscle strain of the paraspinous muscles due to a lousy mattress...or 2. costochondritis-inflammation of the cartilaginous joints of the ribs to your vertebrae, provoked by using your accessory breathing muscles (intercostal) to breathe, since laying down makes your diaphragm less able to move air for you...or, 3. a condition called hydronephrosis of pregnancy, in which laying down, especially flat, allows the weight of your pregnant uterus to partially block drainage from your kidneys, backing the urine up and causing mid-back pain. Of these, I'd put most emphasis on kidney pain, which should be relieved by laying on your side when you recline. If that doesn't help, it's more than likely the mattress.