I had this happen to me as well with my 3rd c/s. It is worth going to the Dr to get it checked out, I had to have swabs done and mine was caused by an infection, but then we used some dissolving stitches to close it up again. I was told it was because I was doing too much and didnt rest enough after the c/s. Not an easy thing to do when you have 2 older DD's who need to be looked after, a Dad that needs care too, a DH who wants his dinner cooked and a baby in hospital and having to do milk runs every few hours.
Get it checked out and hopefully it will be nothing.