What would cause my right foot to swell? Congestive heart failure has been ruled out. No injury to this foot.

DVT. If a deep venous thrombosis has been ruled out then you need to see a vascular surgeon as you may have lymphedema.
Edema one foot. Other possibilities: chr. Venous insufficiency, lymphedema, unlikely small clot below knee, fluid in the ankle 2 to arthritic process (gout), urticariai, allergic to shoe or 2nd to compression of footwear, insect bites, cellulitis ( infection), raynaud syndrome, suggest to see a podiatrist.