Just throwing this out there. A few weeks ago I bought a rode bike and have been struggling with fit issues a bit. Most are really saddle related. Too much pressure in the wrong spots leading to unnerving, post ride numbness. Also, I was having some low back pain towards the end of the rides that ended as I got off the bike.
While playing with fit things, my most knowledgeable buddy suggested it might be a clothing problem. I have been losing weight quite a bit and have still been wearing the same 3x Aerotech bib shorts. I noticed they were getting a little loose, but I thought all fine. My friend suggested trying a size down of the Specialized RBX bib shorts. I got them, they were a little tight on the first day, little more form fitting the next. But my "fit" issues disappeared with only that change. No more numbness, no more back pain doing the same rides.
My daughter noticed that my shorts were a little too baggy when I was losing weight. I also was having saddle issues and part of it was due to baggy shorts bunching up.
Earlier this week I went on a 20 mile ride using a favorite pair of shorts. When I came back, my rump was raw. That's when I noticed that the stitching holding the pad in place was ripping out a lot. I have 3 other pairs of these shorts and they were all starting to rip out also. I'd been having some rump issues recently and the ripped pads could have been part of the problem.
As far as bibs go, because of the recently issues, I've bought some bibs and they seem great except when hot outside. In fact I just bought two pairs of bibs to replace the ripped shorts that I threw away.
I cannot imagine how a new pair of bibs would stop back pain. I suspect that the back pain was due to you first few rides in a new position. Your body got used to it and the cessation of back pain with the switch to new bibs was merely coincidental.