A major rule of fasting is never to break during a crisis! Fast through until you are feeling well again.
Your weight is not a factor. Thin people can fast successfully -- ten days is a good goal though three or five will do you a lot of good. In any case, in the future if you go down, STAY down and drink drink drink! Flush out those toxins and you will come up feeling great! Sounds like you really need a good cleanse.
3-6 glasses of fresh juice, plus plenty of pure water is the rule. Be sure to keep your colon very clean using the colon cleansing ritual of your choice, every fasting day. I have often had a plain water enema stop a detox crisis in its tracks.
Any weight lost during fasting will return quickly post-fast on a good rebuilding diet.