Back labor?

So I’m 34 weeks tomorrow and tonight was the second time I’ve had this unbearable back pain. It feels like hot knives in my lower back and it starts as a dull ache then, over the course of an hour or so, turns into this searing pain that makes me throw up. It also doesn’t come and go it’s constant. And when it happens I can’t really feel any contractions in the front if that makes sense? The first time it happened, we went to the ER just to be safe (FTM here) and because when I was having the pain I couldn’t talk/explain to my boyfriend what was going on so he decided we were going to get checked fearing the worse. Nurse checked my cervix and it was still shut. I’m going to talk to my doctor at my appointment in a couple days but I just wanted to know if anyone else experienced back labor without dilating or could describe what their back labor felt like?