I did really well carrying twins and only gaining about a 1/2-1 pound a week. (I started out quite overwieght and have diabetes) The doctor said I was doing great and I would likely be smaller after the birth than when I became pregnant.
At about the 30th week I stepped on the scale at the docs office and it said I gained 11 lbs in ONE WEEK!!! I said, "There must be a mistake!" I was freaking out!
The nurse said," I can tell by looking at you that it's water wieght". I ended up gaining about 40 lbs of fluid before developing preeclampsia and having an emergency C-section. The scariest part was that for several weeks before the C-section I would wake at 3-4 am feeling like I couldn't breath. As soon as I sat up I was fine. I was one of the rare women who's preeclampsia did not go away after the birth and I ended up back in the emergency room within hours of being released. I had the same thing happen at the same time and something in my brain (or that still, small voice from heaven) said "Do NOT go back to sleep!" Called the doc, went back to hospital (where the girls still were anyway) and was rushed into the ER. I asked the girls who were sticking all kinds of wires all over me and a catheter what was going on with my breathing and they said that I was so full of fluid that it had nowhere else to go and was filling my lungs-essentially drowning me in my sleep!!! That was way scarier than an emergency E-section!!!
Anyway, long story, well, long...That rapid of a gain is almost always fluid and can be a precurser to preeclampsia or a sign of gestational diabetes (as some have already mentioned)
Take care of yourself and it never hurts to prop yourself up in a semi seated position to sleep if you get really swollen!!! (I never woke up when in the hospital because I loved the hospital beds and slept half sitting up)
Reading back over that, it sounds scary, but it is VERY rare for preeclampsia to not end with the birth of baby. In addition, I was an "older" mom with preexisting health conditions and the twins made everything x2, including fluid gain!