Ya that happened to mine. I didn't tighten the collar right and suddenly my BCX2 started to sound really weird, rotate counterclockwise, and not be able to lift beyond about 6 inches. Tighten that collar properly and you'll be fine. Also I clued in when hitting the rudder right and raising the throttle to the point where the upper blades kicked in resulted in the blades slowly spinning up, but the motor RPM remaining constant.
ORIGINAL: whtmex
Also, besides binding, make sure your inner shaft reaining collar is properly tightened. If the set screws on the retaining collar aren't tightened on the shaft flats, the drive gear will spin around the shaft and unbalance the anti-torque effect causing the aircraft to spin.