To add to Noah’s excellent answer, look at where the neck meets the body. You say that the issue is at the 15th fret. On many guitars, that’s right around where the neck meets the body. If that’s the case, you may need to raise the bridge slightly. Typically a truss rod adjustment would be if the issue was presenting itself closer to the headstock, like the first half-dozen frets or so.
But also, if it was a bridge thing, it would usually affect not just the 15th fret, but all of the frets above it, as well. It’s possible your 16th fret runs a touch high, and that raising the bridge a little will account for it. If you’re set on the current action, a tech can examine and level/dress your fretboard. That can be a bit pricey, though.