Your young age, lost of weight, abdomen pain and joint pain might be explained by Crohn`s disease (but it doesn't have to be it). The best way to rule it out is colonoscopy
If it will not show the reason of your symptoms, CT of small intestine
might be necessary as colonoscopy visualizes large intestine only.
Only when this test are done, IBS
might be recognized.
The other option to explain your symptoms is an inflammation of the prostate
gland. There should be any signs of inflammation in your blood tests and urine tests. You should visit urologist
for physical examination of your prostate if this is suspected.