Is there any load and balance and attitude that can put positive G on one wing and negative on the other?
Just imagine an aerobatic aircraft going straight up and rolling at the same trime - sure it will have positive lift on one side and negative on the other while accelerating into the roll or when stopping the rolling motion. Note that during a constant rolling motion while in a vertical climb the overall rolling moment and the overall lift both are zero.

Also, wing twist will cause very little lift or even a small downforce at the wingtip at low angle of attack. I remember that the wingtips of the SB-8 and SB-10 gliders would point downwards from the negative lift at high speed. The high aspect ratio glass fiber wing would bend a lot and indicate the local lift with a local change in curvature.

However, if you were flying an approach, the aircraft should have flown too slowly to allow for that condition, unless it also started to roll violently from the lift asymmetry. If you had to apply full aileron to counter a gust-induced rolling moment, that very aileron deflection would avoid any negative lift on one of the wingtips. This only leaves the possibility that you had negative lift for a very short moment and over a small part of the wing when the gust hit you and before you countered the rolling motion with the ailerons.