There are some things you can do to take the pressure off of your back. You can try pelvic tilts- getting on all 4's and sticking our pelvis out and back in. It's supposed to relieve some of that pain. Also, try one of those maternity belts where it lifts your belly up a little bit....that is supposed to take pressure off your back too.
I'm 19 weeks tomorrow and I'm having the same issue. I have a bad back as it is but the pain is increasingly worse since about week 16. The cramping pains n such I am also having. I am already a high risk mom but even with that being said, the doctor here won't do anything for you until your 20 weeks. Its crap but my doctor said if you're gonna miscarry at 20 Weeks, there's nothing medical we can do. It was like a slap in the face. You should be having your anatomy scan soon which will point out any abnormalities. I just had mine and baby girl is fine, so hopefully you just have a bad back that you didn't know about and the cramping is from sore stiff muscles. Good luck!
To me this doesnt sound normal.maybe the baby is hitting a nerve? I dont really know though maybe it is normal 4some people.id ask your dr