Shifter cable.
It was working fine before you fiddled with it 😉 so it stands to reason that the cable has too much tension, or the outer is too short; it could alternatively be too long, I can’t remember which, but too short would be my call.
To sort it, click down to the smallest cog on the cassette, undo the cable pinch bolt on the mech. This will alert you to any Low Limit incorrectness, so adjust if needs. Check also the B-Tension screw/bolt and adjust as necessary. Put a small amount on tension on the shifter cable and do up the clamp bolt.
Shift up and down a few times and fine tune with the barrel adjuster at the shifter and voila! 😀
Apologies if you knew the process already.