Run it,
I think it's normal to gain 5 pounds or so over the winter months. Why torture yourself about it? And yes, it's totally normal to fluctuate 3 pounds or so depending on one's menstrual cycle too.
It's not good to have only empty skin flap bags as boobs if it's because you are starving yourself. It means you have NO fat on your body at all, and we need some fat to be healthy.
As to the thing you mentioned about the feminine shape becoming more distinct every time you gain and lose weight- when I was 20 -and my 2 daughters as well- we all had very thin willowy body shapes. It was only during our mid 20's that all 3 of us began to get more womanly shapes- with more defined hips, breasts, and thin waists in contrast. It's normal development, and it's meant to be this way. I have some friends that never got hips at all, and they're shaped like little boys still. That's fine, but my genes would never have had that happen, and there's no way I could have kept a willowy figure even if I starved. It wasn't fat, my hip bones just matured!
Hope this allays some of your fears. Don't freak out, try to be kinder to yourself.