I have horrible back pain all the time, i lay on a heating pad every night before i go to sleep... and i also use a little pillow for more back support when im sitting at my desk at work. it helps a little.
I don't want to alarm you... but when I was 15 weeks I had back aches and everyone told me it was part of pregnancy. No cramps, just minor backaches. I had an appt. with my OB already scheduled, so when I went to see her, I found out that I was miscarrying. Not saying that is the cause of your backache, but I would rather you be safe than sorry and give your OB a call. If you describe it to her, she will better be able to determine if you need to be seen or if it is "normal".
My pain is associated with movement. Is that what yours was when you were miscarrying?