Yes!! I had the same issue during my last fast end of July/early August. I only went 72 hours and I had to stop. Knee pain, neck pain, and low back pain. This current fast I am on I have had zero problems with achy joints and I am currently on day 4. I have a theory though. All those areas of pain I had during my last fast have all been areas of past injuries. They say fasting helps to heal tissues. It took me several weeks for those pains to go away even after I quit my fast. But since then they really haven’t came back, and I was expecting that it would with my current fast. I do know that while we are in a fasted state our inflammation markers go way up, some inflammation is good especially with an injury… so what I’m trying to say is, it is my belief you are healing an old injury.