how gain weight

Yuri • Mother of 4
I'm having a hard time gaining weight. I've lost so much weight after my last pregnancy. My clothes fit me loose n I've lost my boobs . I feel hungry shortly after a meal. I'm not starving myself but it looks and feels like I am from always feeling hungry. I'm not sure what is going on with my body but doctors haven't been much of help . Could anyone advise me on my problem please?