WHEN I PUT AN MP3 INTO GARAGEBAND TO USE AS A SAMPLE THE SAMPLE CHANGES WHEN I SAVE AND COME BACK TO THE PROJECT, HELP!

I am using An MP4 that I turned into an Mp3 as a sample on a song. When I try to use the middle part of the file it just changes to another part of the track. When I save the project and come back to it the sample is completely different than when I saved it. HELP!