Back labour?

I've had pretty constant back pain for a few days now. Yesterday was pretty unbearable. Today is had eased up a little, and I felt quite relived. This evening hit and it had flared up but in a completely different way, really intense cramping across my lower back radiating round my sides and down to my hips. Coming in waves. Lasting about a minute but only a minute apart for a good 20 mins. Got a hot shower and sat on my ball for a few hours and it's eased off quite a bit but still having the intense pain but it's just not radiating forward. I can't find much out about if I'm just experiencing normal back pain or if this is back labour? I checked my notes and baby is posterior facing so more chance of back labour, I have a tilted uterus leaning back quite significantly since have had back pain throughout pregnancy. Midwife told me it would feel like period pains but again all my period pains were mainly in my back or the feeling of being very swollen down there. Have had a few miscarriages and the one I was furthest along with (14 weeks) I remember most of the pain was in my back too. Having very mild tummy cramps, barely noticeable. If anyone has experienced back labour I would love to know if this sounds similar? How do I know when to call? 40 weeks today so not entirely surprised if baby is ready to come out.