Muscle relaxers during pregnancy?

My back is killing me y'all. My back got screwed up when I was pregnant with my son, but it didn't start till almost 30 weeks so I don't to stick it out for long. This time around I'm only 14 weeks and can barely walk half the time because it hurts so bad. I called my OB's office but they just gave me the generic "Tylenol and heating pad" answer which does almost nothing for me. I've been doing some research and it looks like there are some muscle relaxers that are class B. Has anyone been on them while pregnant? If so, how did you manage to convince your doctor that it's not just a little soreness that Tylenol can fix? I really don't want to be on unnecessary meds but I can't even be a mom to my 18 month old and it's killing me.