What Causes Swelling On The Top Of Incision Post C-section?

I had c-section 11 days ago. There is swelling on top of my incision. First it felt like fluid inside and now it hardened. My doctor says I need to put heat pack and gave antibiotic to prevent infection. She said if it doesn t heal in 2 weeks, she might have to open it and redo the stitches. What could have caused swelling and how to get rid of it without being opened?