I agree with asking your doctor for full nutrition blood work testing. I've seen some interesting articles lately in the affects of low magnesium (numbers are reported that 70% of the country is deficient) causing sleep disturbances. Iodine deficiency (somewhere around 90% of the population!) can interfere with proper thyroid functions and bromide (aka brominated flour) actually stops the thyroid from absorbing iodine. It's something to look into. I began and ended my pregnancy at those exact weights and even with dairy restrictions the first year have only managed to get down to 135 two years later. I'm not sure of your height but I'm 5'3. I'd be unhealthy skinny at 112. You're definitely right in taking steps to find the cause of this. Here's one link on iodine:
I think the original article I read was from the Dr. Brownstein mentioned in it.