Blood test result was fine. stool test is fine no blood no parasite. why am i still losing weight? 18 years old. can this be cancer?

Multiple factors: stress, tension, anxiety,depression thyroid issues,metabolic dysfunction. Do yourslef a favor and get a comprehensive physicial exam to include comprehensive blood and urine testing. Weight loss deserves much further evaluation.

Not likely: If you are loosing weight , we need to know your diet , activity etc. Do you have any fever , joint pain or swelling etc. Also need to know how much wt have you lost in what period of time , what are your symptoms? You definitely need to be evaluated thoroughly by your physician.