Here's a few ideas
Ensure the clip in the source monitor came from the project bin. ( eg drag it or dble click it from the bin)
Do not double click the clip in the timeline and think that is the source clip when it appears in the source monitor
Deselect the clip in the timeline before editing
Another method is to now work in the Timeline. If you hold down Ctrl and then click-drag on the Head/Tail of the Clip, this will allow you to change its In/Out Points and Ripple the preceding/following Clips to reflect this change.
Good luck, and hope that these help,
Ah...When this happened again today, I double clicked the video in the bin and the problem disappeared...Thank you. Not sure why the original video wouldn't be still in the source monitor, but it worked and I am very grateful.