Answer:
You say you feel healthy your doc says your ok. Don`t worry you have a high metabolism you burn food up quicker than some. You are a mother you`ll be running around doing various things burning up more energy. Don`t try to put it on be happy that you are slim and probably attractive to most guys. Time will come when you will be saying why can`t I be like I was when I was 21, lol.
all I can suggest is not doing any exercise and eating as much fatty foods as you can.
Your doctor is nuts; you need a new one.
You should be in touch with a nutritionist, too. If you're doing your best and it's still not working, you need help. Also, have some labwork done..there might be something there that a pill or supplement can help. Protein shakes and protein bars will help, but make sure that you are also getting enough carbs and fats.
try weight gain shakes! my ex could NOT gain weight until trying those - she didn't gain much, but did gain.
just go to the store and look in the section that has the protein/ diet stuff - look for the mixes that say that they are for gaining weight - then drink LOTS.
Peace