Lower back spasms at 36 weeks? False labour?

It's hard for me to say because I have a previous back injury that's been bothersome, but the past few days I've had these completely different back spasms. It feels like my lower back tightening over and over and it's quite painful. Lasts maybe 40 seconds at most. Initially it was once a day and now it's happening a few times a day. Can you have "practice contractions" in your back? No abdominal cramping.