I'm a little older than you and a year out of Nissen. I have lost some weight, slowly at first, but continuing. I look a little skinny now.
I have to make sure to eat breakfast, a little mid morning real food, light lunch, afternoon something worthwhile, light dinner and often an evening snack. I sometimes wake up hungry in the middle of the night.
Our stomachs are considerably smaller than they were pre-op. Food may stay in them a shorter time, may traverse the small intestine a bit fast too. We need to make sure we are getting enough nutrition and proper supplements. Ask the doc for blood tests to see if you are anemic, have low vitamin B 12 and D3 levels. I need to supplement with iron, B 12, D3. Supplement with calcium, but don't overdo as it is now shown to cause buildup in arteries.
I have lots of trouble with gas, gassy foods, many of which are the vitamin K containing foods. I do tolerate a little salad which also has K.