TOO underweight for 30 day fast?

Dear all,

about 3-4 weeks ago i did a 10 day fast and dropped to sub BMI of high17, low 18 (not a big deal).. I had a good few weeks of re-feeding but seemed to not put on the weight, as I was pretty cautious with what ate, broke my fast over a few days, and continued with internal cleansing every 3-4 days. If there was a special treat like home baked goods (organic, no refined flours or sugars) I would split it up over days.. so obviously careful moderation was taken.

I was about 51kg (112) 168cm (5'5'') going into the new fast (2nd oct) - which is technically underweight but no too bad... My re-feeding didn't really bulk me up though. I have not weighed myself and am wearing lots of baggy warm clothes (avoidance much??) . I have noticed that I am loosing weight more rapidly than in my previous fast. I also noticed that It was hard for me to gain any substantial weight in refeeding..

at what weight is it dangerous for me to continue to fast?
Is there a point that I should stop fasting? or should my body somehow cope and find a way to stabilise my weight?

any info/experience would be appreciated. I am not worried for myself, and I know when and how to break if it gets bad, but I would like to re-assure my family as they are seeming albeit supportive, but quite concerned.

Praise to God.