We need a lot more info than that to help you properly diagnose your problem. What are the specs of your system, your project, and the footage you're working with?
Welcome to the forum.
As David asks, more info will be useful. Please pay special attention to furnishing the pixel x pixel Frame Size of your Source Footage (also list the PAR - Pixel Aspect Ratio), and that of your Sequence's Frame Size. There is very likely a mis-match in there somewhere.
Also, the Source Monitor just displays the Source Footage, irrespective of the Sequence/Project, in which it will be used. The Program Monitor takes that Sequence into consideration, and will display it, relative to that Sequence.
Good luck, and please let us know a bit more. I feel that the solution to your problem will be in those answers.
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Most likely you're putting your high definition footage into a standard definition sequence.
Don't do that.