Quite often a baby will wean on their own if they are feeling satisfied with their own food intake.
However if you still offer the breast milk, then she won't wean right? I wouldn't be too concerned about weaning her until you are satisfied she is getting all the nutrients she needs from her diet. With in reason of course cause you don't want to be breastfeeding an 8yr old. :) *joke*
You said you were only nursing at night right? Most times when the woman is down to nursing only once a day menstruation will return.
Dietary changes can in fact alter your menstrual cycle, as can actual weight and weight loss.