Honestly, the only thing that gave my back relief during pregnancy was 2 extra strength Tylenol and a heating pad. I remember the pain was sciatic type pain that went into my butt cheek and down my leg(s)...UGH, it was horrible! The heating pad gave me some good relief. Also, talk to your doctor about what other medications could be safe to use at your stage of pregnancy. Since you are in your second trimester, some medications could be okay to use now, unlike the first trimester. Just an FYI...if your back pain becomes severe, please contact your doctor immediately or go to the E.R. Depending upon the severity of the pain, it could be something that would need immediate attention (think kidney infection, etc).
I hope you're able to find some relief. I know when your back hurts, you pretty much hurt all over. Good luck and take care!