I don't have CF, but there was some weird kind of bug going around here during the holidays. I started getting lower back pain, but it was more of an ache, not a sore muscle and I was thinking it might be a kidney issue.
This went on for a week and turned into a full blown back spasmy issue. But here's the weird part -- two of my coworkers got the same exact thing -- not in the same area, but a weird ache that turned into muscle spasms.
All I can figure is there was some sort of bug going around and because we were favoring that area of the body, other muscles started compensating and then spasms occured. Not sure if it's something similar, but I have never ever had anything like this before and then it just went away as mysteriously as it began.