Like you I had a vaginal birth the first time and a c-section the second (singletons).
I totally felt the way you did about my belly after the second, and blamed the c-section. I just felt like it was all puffy and loose in there, like they didn't stretch it tight when they sewed it up, ugh. It bothered me for over a year. It still bothers me a year and half later, t hough it is better now. I tend to think it was the effect of a c-section. Then again, maybe our tummies just have a harder time going back to normal with each additional pregnancy pop-out? Not to mention I spend much less time working out with two kids in the house than I was able to with only one. So who knows.
I know what you mean though, I felt like my stomach was totally different after the c-section and was really bothered by it. Don't remember having any issues like that with the first, vaginal birth. Having a c-section just felt so unnatural to me, that probably adds to my blame.