Sxt, it's difficult to say what we think you have, because there are a million and one different disorders that can cause the symptoms you are experiencing. IBS is the most common of those disorders. Being 21, it is HIGHLY unlikely that any kind of cancer is present, and you really should not be worrying excessively about
that possibility. The anxiety will drive you batty and probably make your symptoms worse! Your appointment is on the 27th and that's not too far away (many people have to wait several weeks or months for specialist appointments, so you're actually pretty lucky)! You could speculate all you want about
what you have or don't have, but until you see a physician and have a few tests done, it is fruitless to worry yourself!
Like dbab mentioned, ulcers are not caused by stress; they are caused by a certain type of bacteria called Helicobacter pylori, or can be caused by overuse of NSAIDs like ibuprofen, naproxen, etc. So tell your friends to bug off about your gut problems, and go play frisbee with them!