What would cause severe nausea and vomiting 8 months after gastric sleeve?

Greetings from Dr HE,
Thanks for your question.
Its seems gastric sleeve is too narrow. Endoscopy should be made. Then the best will be made for you.
All the bests 
Dr HE

Without knowing the results or findings of the investigations you listed above (EGD and stents) it may be worth looking into a contrast study to see where the hold up/obstruction level it. Is it that the sleeve is too tight and therefore not emptying properly? There is one area in the stomach in particular that extra care needs to be taken during a sleeve gastrectomy, in order to avoid narrowing of the sleeve.

after the sleeve you may need to close some scarring area and produce symptoms such as these.  This surgery also cause reflux symptoms. Other possibility could be an active gastritis, helicobacter pylori or gastro esophageal reflux. At this time, these symptoms should not be present. Possibly some changes are important feeding, use of medications that reduce gastric juices and others that make emptied faster. The most important thing is that you quickly go back to her surgeon or lead her team support. soon she'll be better if you attend with your physician

It sounds like her sleeve is too narrow.  There are several options.  The first is to try to wait it out, hoping that it will stretch out on its own.  Next would be to try to stretch it endoscopically, or to place a stent to try to stretch the stricture.  If those options don't work, surgery is the next step.You can convert to gastric bypass, which would bypass the narrow part of the sleeve.  This is the most reliable choice.  For patients who really don't want a bypass, there are a few other "less guaranteed" options.  One would be to try to widen the narrowed segment by cutting the outer muscular layer of the stomach.  This is called a seromyotomy.  Finally, you could cut out the narrow segment and sew the sleeve back together.  I've seen all of these methods work successfully.