Thank you for the additional information.
It would be very unlikely that you have cancer as the cause of the weight loss. It would be very unlikely for someone to have stomach or colon cancer at age 30. We typically do not consider a CT scan as a sufficient test to exclude the diagnosis of stomach or colon cancer, as the tests that would be most effective at excluding the possibility of stomach or colon cancer would be an upper and lower endoscopy. However, the vast majority of people with stomach or colon cancer that has progressed to the point of causing weight loss would have abnormalities seen on the CT scan.
It is still likely that the weight loss over the last 3 weeks is related to the previous changes in your diet. Since you had been having progressive weight loss because of a dietary change over 6 months, it would have induced changes in metabolism, so that the body is more oriented towards breakdown of stored calories, called a catabolic state. Once the catabolic state has been induced with a healthy diet and exercise regimen, it will continue for a period of time when you resume eating a regular diet.
However, when someone has ongoing unintended weight loss, there are many conditions that should be considered. It can be related to conditions that affect the ability to digest food, which can include liver or pancreatic dysfunction, inflammatory bowel disease, or malabsorption syndromes, such as celiac disease. Fatty infiltration of the liver will usually not cause liver dysfunction, although a small percentage of people can develop associated inflammation of the liver. It can be related to metabolic conditions. The most common are diabetes and thyroid disease, which would have been excluded by the blood tests that have been done, but it would be appropriate to also look for adrenal disease. It is also common to see weight loss with certain mental health disorders. We see this most often with depression, but it also can be related to anxiety.
At this point, there are a couple options. It would be fine to give it some more time to see whether the weight stabilizes as the body adjusts to the return to your normal dietary intake. However, if you would rather pursue further evaluation now, the next step would be consultation with a Gastroenterologist, who can assess for the various GI conditions noted above.