So at first I thought you meant something else, but what I misunderstood is still really interesting.

An app where you can just list your symptoms and how they change over time and then gives you a percentage chance that you may have any given condition. Kinda like... "My back hurts, it's been hurting for a week and getting worse, my skin feels dry, had a headache three days ago..." and the app tells you you might have some this-or-that infection.

I don't know how doable this is at all.

Sorry but I don't know about your particular issue... I would recommend just using Sheets? I mean, based on what you're saying you just want something where you can easily input a number (pain level) or symbol (took meds/didn't take meds) over time on a few different axis and get a visual representation (graph). Why not just use Sheets?