I notice for some reason after I speed up some tracks in my movie, the application gets into a mode where instead of adding a object to my track it starts to replace the last object in my track. I'm forced to copy and paste the last object then move the cursor after the last object and add a new object which it replaces with the last object.
It sounds like you are in import Swap mode (shortcut 4). Select a video/photo to import, press the number 1 on the upper row of your keyboard. This is the shortcut for Automatic import which will put the object at the end of the timeline. The mode will now be Automatic.
If you don't know what the various import modes are, I suggest that you watch my tutorial on Basic Editing Part 1.