Am 34 yrs with a lost of appetite, losing weight, with nausea and a painful swelling behind my knee up my leg and am losing weight?

Need to see a doctor. While the symptoms you are describing are not specific to any one diagnosis they are concerning enough that you should see a physician as soon as possible for a formal history, physical exam and possibly lab tests and imaging.
Deserving. You seem to be deserving of a thorough medical evaluation given your concerning symptoms. The swelling behind your leg may be worth imaging with a ct scan or MRI and if the mass is suggestive of a tumor (sarcoma), then additional scans of your lungs and a biopsy would be in order.