I have never had the surgury, but I can see how hair loss could be a side effect. Doesn't the surgery grossly limit what you can eat? That really can effect hair growth and retention, I know.
A few years ago I lost 15 pounds in a very short time using Diet Fuel with ephedra. The supplement totally wiped out my appetite and I was NOT eating much at all. My BF started to accuse me of having a eating disorder it was so bad. I lost the weight way too fast. One thing that happened to me during that time was hair loss and my teeth got weaker for some reason. I know it was because my body was not getting the proper nutrients.
Maybe your doc can give you some advice on the things you can eat that will give you the most nutrients possible. Also he/she may be able to tell you if your body will normalize and get used to the change in your diet. Will you be able to take vitamins?
I mean, girl even my hormones change the way my hair grows. I get a growth spurt right before my period and then right after it is over I get lots of shedding and then it stops.
Maybe any hair loss will stop after your body gets used to the change.