Swelling in the hands, feet, stomach is called anasarca. The most common causes of swelling in the body include hormone disorders (thyroid), protein deficiency, malnutrition, kidney diseases, liver disease, or
medications. So far, you have had the basic tests for ruling out congestive heart failure and lung cancer as causes for your edema (fluid retention). However, make sure these lab tests have been done to rule out the other disorders listed above:
1) Liver function
2) Kidney function
3) Thyroid function
4) Protein level
Your medications need to be looked over thoroughly to rule out any possibility that they can cause your fluid retention, too.
I would follow up with your health care provider so that they find out why you are swelling.
Regarding your questions about cancer causing this, I do think that cancer needs to be ruled out. Certainly abdominal cancers, especially liver cancer, could cause edema. Prostate cancer should be ruled out as well, although this would be a less likely cause of your edema.
Also, please get some help for yourself regarding your depression. Again, you need to have your thyroid hormone checked to be sure this is not why you are depressed, too. Remember that you are still young and have alot of life ahead of you. You need to get healthy in body and mind to live a good life, as they are related, and you cannot have a healthy body with an unhealthy mind.
My last thought is once you are feeling better and your depression is under control, you should try to quit smoking, as this would be another positive thing you can do to be more healthy.