Okay. Now. First question: why is that number so important? If you could wear, say, a size 8 and still be 190 pounds, still have to look at that number on the scale BUT still have that number on your clothes, would you take it? Not arguing, just asking.
Second is a statement: one pound of fat is about 3x (or is it 5x?) as bulky as a pound of muscle. So if you've LOST 5" and your weight has stayed the same, you're taking off fat (getting smaller) but putting on muscle (so the weight # is the same). The body eventually stops doing that, without the help of steroids at least, but right now you're in that "let's build muscle!" zone.
There is the possibility that you're not eating enough. When you have more muscle, you need more protein. What's your diet like?
About your sore back: add some core work (crunches, woodchoppers, trunk extension) to your workout. When your core (stomach and lower/mid back) are stronger it takes the stress off your other muscles. They're sore because they're doing some of the work of your weaker core. Try Pilates, as well. Pilates rocks.