The first thing that I woudl check is that you didn't bump your elevator trim. If you did, reinitialize the gyro after putting the trim back in the center. If that is not it, then you will need to put a swash leveling tool on the helicopter and level the swash in DIR mode (given you are running a 3GX). This should take that drift out of it. Somehow, it's seeing a forward cyclic movement and pitching the nose down. Keep us posted.