Potatoes do cause inflammation for me, as do tomatoes. It looks like I will probably have to avoid nightshades (or at least those two, and I never ate much of the others), which is such a bummer because I love them!
But I get these painful red patches of inflamed skin when I eat them. And I started having this grinding pain in my hip that my doctor said was likely to be bursitis. I stopped eating potatoes and tomatoes and both have cleared up totally. I experimented by adding them back in, and both conditions returned.
I would guess that some people are sensitive to them, and most aren't. You'll have to experiment and see how they affect you.
Out of curiosity, do you get those reactions with ORGANIC potatoes and tomatoes? I've experienced "reactions" to certain foods that have nothing to do with the food, but a certain preservative or perhaps chemical that was on it. Eating the organic version produces no such reactions.
This is why the only way to know for sure is to do a strict elimination diet over time as Burgess has done and, might I add, is still doing and still learning.
This is a very complex situation and most likely not the nightshades, per see, as if it was the nightshades, then there are other nightshades that would have to cause the same reaction.