Congestive heart failure (CHF) is the most common reason for hospitalization in people aged 65 years and older in the United States. Especially, a very high rate of unplanned readmissions within 30 days of discharge due to CHF occurs a heavy associated financial burden and degraded quality of care. To effectively prevent CHF readmission and improve overall care, this paper proposes SwellFit, a novel wearable sensor that helps outpatients to proactively monitor a physical symptom of worsening CHF, ankle swelling. Using a flex sensor, SwellFit monitors changes in ankle curvature as an indication of swelling. A pilot test with 4 adults showed that SwellFit successfully distinguish noise data and motion artifacts from ankle swelling. Collaborating with cardiac experts, we are currently planning to test SwellFit with hospitalized CHF patients who have swelling conditions to demonstrate the feasibility and reliability of SwellFit on detecting swelling from ankle curvature.