Hi bella! I had a bad back on my first fast, I couldn't work it out because bad backs usually happen when you put on weight not when you lose it!
Anyway I think it was Chris or Fonty who told me that it was not uncommon to suffer back pain when fasting, and it may have been my posture re-adjusting. The other explanation given was detoxing in the lower back area is common. But I put it down to the first one in my case, because after that fast I was half a centimetre taller! No more back pain since then, and my spine feels much straighter compared to what it used to be like.
Of course this may not be the same for you, especially because you are about half a ton lighter than I was on my first fast! On the other hand, the percentage weight you lost in terms of your whole body was probably higher than mine, so maybe it was that, or the detox thing.