Try wearing a maternity belt hun. $19 at walmart. Helps support back and belly.
You probably have pelvic girdle pain it where your hips loosen too much the pain is so sore I have it and it hurts to turn in bed get up from sitting and walking you should see can you get a referral for a physiotherapist and if you can't get exercise ball and roll hips and back it's really sore at first but it really helps another thing use hot water bottle when sitting or lying helps loads!
Your body is adusting to the extra weight from carrying your baby, especially since your baby is gaining more weight at a quicker rate. I would take tylenol and maybe enjoy a warm bath or shower or use a heat pad?
Maybe see if your insurance will cover a chiropractor. I honestly don't think your back should hurt that bad mention it to your doctor !