I'm having my third pregnancy and it's not like the first two at all. …

I'm having my third pregnancy and it's not like the first two at all. For a whole week or even more, I just have this constant pain in my lower back and it feels like pressure, and I don't understand why. It's already 39 weeks. One day, I had my first baby at 38 weeks, and for my second at 37 weeks. Maybe someone has experienced something similar? Sometimes, does it feel like a strong pain in the lower back that lasts for a long time? Like when contractions start, I think it's labor but then it passes. At the same time, I have muscle tension since 20 weeks. Thank you for your answers.