Help with 2d animation!

Hey guys! I was trying to learn grease pencil 2d animation today. It was all nice, but now the last keyframe stays always on! i don't know if I explain myself, what im trying to say is my last keyframe was on frame 300 of the timeline, now when i try to draw on frame 340, whatever was drawn on the last keyframe (frame 300) keeps appearing, to put it other way: I have 41 keyframes, whenever i draw my 42nd keyframe what i drew on frame 41 stays in the keyframe 42, and if i try to add another keyframe after that by editing or drawing another stroke frame, 41 keeps there, any help? this is not related to onion skinning. Any help is apreciated!