First of all, you might have animated the anchor point. If you didn’t, make sure your position path is not curved between these two keyframes. This might have happened if your path is curved on the previous keyframe, so when you set a keyframe a little further down the timeline, it’s automaticly going to curve it in the opposite direction. To fix that you might want to try the following: Cmd-click (control-click on windows) on the handle that’s on the opposite end of the curve (to seperate it from the other handle) and drag it towards the keyframe (till it’s on the keyframe) that way it takes away the unwanted curve, than use the other handle to fine-tune your previous curve.
I hope this is the solution,
-A.N.