I take voltaren (diclofenac) 2x/day for chronic low back pain. But I also have swelling in hand/foot/knee that it doesn't help. I'm late 30's.What else could it be?

Voltaren (diclofenac) I would go see a doctor if I were you. What you got could be due to many different things, and more pertinent history such as would be necessary to throw some possible causes at you. Rheumatoid arthritis, lupus, arthritis due to inflammatory bowel disease or chronic hepatitis, or complex regional pain syndrome/rds are some of the possible causes. Voltaren (diclofenac) could be bad for your heart and kidneys.
Why back pain. Your evaluation needs to clarify why you have back pain. There are inflammatory back disorders such as ankylosing spondylitis that can respond to voltaren (diclofenac) and this inflammatory condition may be related to the swelling you are describing.
Side Effect? Swelling in the hands & feet could be a side effect of voltaren, (diclofenac) which can cause fluid retention, or excess protein loss in the urine, resulting in edema, or swelling due to having too much fluid in the tissues.
? inflam. Arthritis. Get checked for rheumatoid arthritis and lupus screen with sed rate.