I don't think you felt movements that early, even 2nd and 3rd time moms who feel their babies earlier than ftm only can feel them around 13 weeks. Plus baby doesn't even have limbs yet, just little nubs and their little flipping we feel as "fluttering" isn't felt in your back, just your lower abdomen. Honestly it couldve just been some slight cramping/muscle twitches. Throughout pregnancy your hips start expanding and a lot of women experience ligament pain, so you could've just felt something light like that.
I feel fluttering in my stomach sometimes and I'm only 9 weeks 3 days 3rd pregnancy.. They say every pregnancy is different. But idk about feeling it in your back.. I just have back pain a little..
I've never felt early movement in my back. But I could feel tiny movement every now and then starting at 10wks. This is my 3rd..