Hi Ipren and welcome to 3FC!
I have not had surgery so I am not sure what your diet restrictions may consist of per your doctor's orders but maybe you are not eating enough calories in a day? You need to be sure to fuel your body.
For advice and tips on different eating plans, you should check out the Diet Central here -
http://www.3fatchicks.com/forum/#diet-central
And there's a forum for weight loss surgery here where maybe someone can relate with your story and needs -
http://www.3fatchicks.com/forum/weight-loss-surgery-78/
Good luck with your goals.
Hugs
Michelle