Hi C.,
I got very sick with a respiratory infection in my 3rd trimester. It was so bad I ended up hospitalized for a full week at 39 weeks pregnant. They discharged me the day before my due date.
I would keep the lines of communication open with your doc. Mine didn't fully realize the magnitiude of my condition until my pulminologist intervened. Once in the hospital, the guys who gave me breathing treatments told me that it is VERY common for women to get respiratory infections while pregant and that they see a lot of pneumonia.
One thing you can take that helps is steroids - they are safe for the baby, so it may be something you want to discuss with your doc. I coughed so hard that I cracked a rib, which was really no fun.
Also, discuss the pain with your doctor. Tell him/her how much you are worrying and stressing about it - stress is not good for you or your baby, so maybe they will pay better attention.
As far as not being able to keep anything down - my doctor explained it this way. It's a horrible way to say it, but babies are like parasites. They will take and get whatever they need from you to survive, even at the expense of you. So while you may end up really depleted, your baby will continue to use what your body naturally has in reserves as far as nutrients.
Please don't worry and keep doing what you are doing - keep drinking as much as you can, eat what will stay down and keep pushing your doc on the respiratory stuff.
I can tell you that despite almost three months of horrible illness and MASSIVE doses of steriods, I have a healthy, smart, beautiful baby girl.
C.