When you dragged and extended the clip in the Arranger, you actually looped it several times. SO when you edit it the changes will take place in acginstance of the loop.
If you want to treat it as a single long clip, then, in the Arranger, select it and Glue it (make sure no other clips are selected.
Below, I have a single 2 bar clip on the first track. On the second track I dragged it out to loop 4 times. Notice the small v-shaped indents every time it loops. On the third track I Glued it to create an 8-bar clip.
If you want to extend the clip without repeating the notes, then turn off "Loop Item source" in Item Settings. Then drag the right-hand edge of the clip.