Can blood test find any problems with my digestive system? also can doctors find a cancer or any other serious problem with blood tests?

Possibly....: What type of digestive system problems or cancers are you concerned about? Are you having any particular symptoms, or a significant family history of a certain type of cancer? It is never wise to run tests for a random reason. They can often have "false positives" - tests result positive, but it's an error. This can lead you down a very frustrating road. Discuss with your doctor. Best wishes.

Many issues: There are many issues that can be diagnosed from blood tests from the more common like H.pylori infection and celiac diease to the very rare. But blood tests are not the end all and be all, they are only a small part that is helpful in making a diagnosis. If you are particularly concerned please speak your with your primary care doctor to come up with a plan of action. Best of luck.

Blood tests: Blood test can sometimes help with GI diagnosis. But its not specific for cancers as such. GI cancers are very rare in 20 yo patients. H. pylori infections are much more common. Ulcers from ibuprofen/NSAIDS is a common issue. Celiac disease etc would be a concern. Avoid alcohol.