Are you eating the potato skin, or just the white "meat"? The skin could certainly aggravate IBS, but generally the white part is fine. Maybe you have an allergy to potatoes. I thought they were bothering me too for awhile, but then it turned out I was often eating them with red meat, which I later found out was an IBS trigger. And here I was blaming the taters!
By the way, before you get too excited about surviving last night's dinner, please bear in mind that sometimes you could get a delayed IBS reaction. Often I get sick the next day or even a couple days after eating something. That's what makes it so difficult sometimes to control IBS and to figure out what is causing the discomfort.