additionally, I would take an Epsom Salt bath. (and be careful with getting out, and getting back to bed;it can make you feel quite like a soft noodle.)Most of us are sorely deficient in magnesium. I do not consider that 'breaking the fast'. Low back pain is a common thing while water fasting, according to the old fasting masters. Seems that a lot of toxins settle there--and could be related very much to the colon, etc. Have you tried EFT? It's a huge help for the ups and downs of fasting, and that's just the tip of the iceburg.
Interesting that it reminds you of labour. Assuming you've been through that, it might actually be healing something related to that.
try to tough it out. (although EFT can dissolve many pains) Do rest as much as you can.
Here is Gary Craig's new EFT site. He is the founder and retired a few years back. His work is impeccable. Free manual, etc. http://www.garythink.com/