Backaches are pretty common by 28 weeks, honestly. Your body's shifting weight forward and your ligaments are loosening up to prepare for birth, so it's a lot of strain on your back and legs. I found that sleeping with a pregnancy pillow between my knees really helped, and doing gentle stretches or even just walking helped more than staying still. The tiredness is real too because your body is working hard growing a human, so try to rest when you can. If the pain is sharp or gets worse, might be good to check in with your gynae or PD just to rule out anything else, but most of us go through this in the third trimester.