13 weeks and losing weight.

I went to the doctor yesterday just for that monthly check up and what not for myself and the baby. Baby's heartbeat was strong at 150. But the doctor is concerned about me. I've lost 5 pounds since my last visit. He asked if I have been holding my vitamins down okay, which I have... just I get sick multiple times throughout the day everyday. Does anyone have any advice on how I can put on some weight?