One keyframe affects the next one (but none after that)

(With animate on) It does this whether it is a ball with a peg, or a complex rig- if I make a keyframe on frame 1, go to frame 2 and translate, or rotate then go to frame 3 and translate or rotate its fine. But if I go back to frame 1 and translate or rotate, it suddenly will add that rotation or translation to whatever I made on frame 2 (the very next keyframe) but frame 3 will be fine. Any suggestions?