3 years post and I started throwing up a lot. Most of the time it's mucus. Today it was blood and mucus. What could be wrong?

Hello,
Thank you for your question. I’m sorry you are experiencing this. There are many possible explanations for your vomiting. You will need a formal evaluation by your surgeon or primary care physician with possible endoscopy (camera down your esophagus into your stomach) and radiographic imaging to determine the underlying cause.
Best wishes,
Dr. Moein

Three years out from a gastric sleeve, vomiting—especially frequent and involving mucus or blood—is not something to take lightly. While occasional nausea or reflux can occur even years after surgery, persistent vomiting often points to a more serious issue that needs medical evaluation.
When patients report vomiting mucus, it can be due to chronic irritation of the stomach lining or esophagus, sometimes caused by reflux, a narrowing of the sleeve (known as a stricture), or even delayed gastric emptying. However, the presence of blood mixed with mucus raises more concern. This could indicate inflammation or ulceration, possibly in the remaining stomach or esophagus. Repeated vomiting can also irritate and tear the lining of the esophagus—a condition called Mallory-Weiss tear—which may lead to blood in the vomit.
It’s essential that you’re seen as soon as possible. Continued vomiting and bleeding, even if small amounts, can quickly lead to dehydration, nutritional deficiencies, or more serious complications. An upper endoscopy is usually needed to diagnose the exact cause. In the meantime, avoid solid foods, stay hydrated with small sips of water or electrolyte drinks if you can keep them down, and seek urgent medical care—especially if the vomiting persists, the bleeding increases, or you feel lightheaded or weak.
This isn’t something to try to manage at home; it needs professional evaluation right away.

It could be lots of things.  There is no reason to speculate.  See your doctor and get an endoscopy.