First, I would say that if it causes you concern, you should talk to your OB about it.
That being said, my last C-Section was 6 years ago (on Tuesday, in fact) and there are still odd feeling, tender and numb areas along my "rope" as I refer to it, because it feels like there is one under there. In the normal course of trying to feel slimmer and tightening those muscles to "suck it in" the incision line will sometimes cramp up terribly and then eventually relax and go back to normal - eventually being less than a minute, which can feel like an eternity sometimes.
Best wishes to you and congratulations on your 3 1/2 month old.
B.