I am a 35 year old male and I have the exact same problem. After eating greasy food like things sauteed in butter or pizza, I will "burp" up large quantities of pure, crystal clear, orange grease. Nothing else, just grease that looks exactly like the stuff that pools on top of a pepperoni pizza. I, however, am not a healthy weight. I weigh in around 325 and have type 2 diabetes and high blood pressure. Here is the kicker thought, My cholesterol is PERFECT. Bad is non existent and good is right where it should be. It's like my body has a filter on it and after the food has passed, the "bad" grease comes back up.
I have the same exact problem, and am around the same age. Strange.
Two things. One is if the pancreas is not working right, for example becuz a duct is blocked for some reason or the gland itself is sick somehow, then since the pancreas breaks down fat, could be it's not working properly, which a gastro doc can determine by ordinary office exam, or an endoscope or scan test. But what is more likely is that greasy foods are always hard to break down, because the human body is not set up to take straight liquid fat and digest it. So, whatever it is you're eating that is greasy, STOP eating it, period. Find something else you like besides greasy food. You can always fix that type of food at home, but use a scant amount of low-fat vegetable or olive oil to cook it instead, check the labels and compare the amount of "saturated fat" that is in the bottles, the less the better.
Take for example french fries from the fast food place. A couple of those types of companies (not all) cook french fries that a single serving of those will have.....drum roll.....1,000+ calories in it. Imagine that. People normally consume something like 2,000-4,000 max calories daily, so if 1/2 to 1/4 of your daily food was one serving of french fries, well, that's how come we got so many obese people in the U.S. of A., too many visits to the fast food place, all that fat, or too much fat from ANY source, can eventually trigger a wierd metabolism problem and folks cannot burn calories like they used to. And even the fast food places that have finally begun to use lower-fat oil to cook the french fries in, there's still a substantial amount of calories from fat in french fries. Of course, it's not just french fries that have lots of fat in them... in other words, if you stop the french fries but keep eating triple burgers with mayo and chili, you'll be getting too much fat if you eat there a lot.
But I think if you will try not eating greasy foods, you won't have nausea problems anymore. I mean, I have to wonder how YOU could wonder what's wrong, when if you "spit up pure grease," that pretty much speaks for itself. I mean, I'm getting pretty nauseas just thinking about it.