Why am I suddenly weak? Please help?

You are not consuming enough calories. A man of your size should be consuming at least double that amount of calories. You are getting weak as your body is burning ur muscle as fuel and you are cold as your metabolism has slowed down to save calories. I know this is hard to consider but you may have an eating disorder, Automatically you will disagree but at least think about it and talk to ur gp about it. You cant live on 1200 calories a day and if you allow urself too an eating disorder will take over you and become you.