There are many causes of malabsorption
. Often, despite extensive workup, the underlying cause is not determined.
Although I cannot say for certain that you do not have lymphoma, I can say that it is unlikely that you do.
You are 27, and have had gastro-intestinal symptoms for a year and a half, without any other "classic" signs like fever, weight loss, sweats, itching etc. This makes lymphoma a less likely diagnosis in your case.
Please consult a good GI physician.