It almost sounds like the diet you were on previouslly was unhealthy for you. Those also sounds like the effects of anorexia, ie feeling cold, period stopping, no energy.
The right weight for you may just be the weight you feel best at, but many use a bmi calculator to figure out what their target should be. Though bmi calculators can't be completely relied upon as they don't take into account how much muscle you may have.
I find this site has good imformation about the calculators themselves and has a few of them
http://www.halls.md/
When I started I had a bmi that stated I was obese and now I have one that just states I'm overweight
I'm shooting for 140 because that will be in the normal range.