Your protein powder shouldn't give you any issues after gastric bypass surgery. High protein, low calorie powders (which we would be able to make for you) are often used after bariatric surgeries. You may need to vary the amount of liquid you use and sip the shake over a longer period of time, since you will have a decreased gastric capacity. Generally, very small and frequent meals including high quality protein is the intended diet, at least for a certain amount of time following surgery.
We always recommend consulting with your physician before starting any new dietary supplement, especially if you have any pre-existing medical conditions or are taking medication. Your doctor will likely have a more detailed suggestion for you.