Hi, KKitty. I have chronic pain and chronic illness too. I've done a couple Whole30s and a lot of "Whole-30-ish" long-term eating. Though I do some coaching, it is not health coaching, so please don't take my post as professional nutrition advice!
That said: if the Whole30 helped you notice nightshades, that's a great thing. If I were in your position, I'd turn this into a Whole60 or at least a Whole45, and eliminate the nightshades, too. Shannon suggests something similar. Then do the reintroductions—slowly, and not starting with nightshades. Reintroduce lentils, wait three days, etc. Do that for a whole month, then try a potato.
Note that some people react to some nightshades but not others, or that our reactivity goes up and down. One of my children is currently in high reactivity due to allergies and mold exposure—and is suddenly allergic to potatoes. Tomatoes seem to be fine. When I had nightshade reactivity, it was eggplant that knocked me out. So anyway, if you do your reintroductions slowly, you may find that after 30-60 days of being off nightshades AND other problem foods, you're able to have one or two nightshades no problem.
Honestly I think most of us with serious chronic inflammation have to get to about 3-4 months of "Whole30-ish" or hardcore paleo/W30 before we can really get good elimination diet results. So it may just take a while.